diff --git a/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlDecoder.java b/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlDecoder.java
index 4bba366e44..881460a49a 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlDecoder.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlDecoder.java
@@ -16,11 +16,13 @@
 package com.google.android.exoplayer2.text.ttml;
 
 import android.text.Layout;
+import androidx.annotation.Nullable;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.text.Cue;
 import com.google.android.exoplayer2.text.SimpleSubtitleDecoder;
 import com.google.android.exoplayer2.text.Subtitle;
 import com.google.android.exoplayer2.text.SubtitleDecoderException;
+import com.google.android.exoplayer2.util.Assertions;
 import com.google.android.exoplayer2.util.ColorParser;
 import com.google.android.exoplayer2.util.Log;
 import com.google.android.exoplayer2.util.Util;
@@ -32,6 +34,7 @@ import java.util.HashMap;
 import java.util.Map;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
+import org.checkerframework.checker.nullness.qual.PolyNull;
 import org.xmlpull.v1.XmlPullParser;
 import org.xmlpull.v1.XmlPullParserException;
 import org.xmlpull.v1.XmlPullParserFactory;
@@ -110,18 +113,18 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
       Map<String, TtmlStyle> globalStyles = new HashMap<>();
       Map<String, TtmlRegion> regionMap = new HashMap<>();
       Map<String, String> imageMap = new HashMap<>();
-      regionMap.put(TtmlNode.ANONYMOUS_REGION_ID, new TtmlRegion(null));
+      regionMap.put(TtmlNode.ANONYMOUS_REGION_ID, new TtmlRegion(TtmlNode.ANONYMOUS_REGION_ID));
       ByteArrayInputStream inputStream = new ByteArrayInputStream(bytes, 0, length);
       xmlParser.setInput(inputStream, null);
-      TtmlSubtitle ttmlSubtitle = null;
+      @Nullable TtmlSubtitle ttmlSubtitle = null;
       ArrayDeque<TtmlNode> nodeStack = new ArrayDeque<>();
       int unsupportedNodeDepth = 0;
       int eventType = xmlParser.getEventType();
       FrameAndTickRate frameAndTickRate = DEFAULT_FRAME_AND_TICK_RATE;
       CellResolution cellResolution = DEFAULT_CELL_RESOLUTION;
-      TtsExtent ttsExtent = null;
+      @Nullable TtsExtent ttsExtent = null;
       while (eventType != XmlPullParser.END_DOCUMENT) {
-        TtmlNode parent = nodeStack.peek();
+        @Nullable TtmlNode parent = nodeStack.peek();
         if (unsupportedNodeDepth == 0) {
           String name = xmlParser.getName();
           if (eventType == XmlPullParser.START_TAG) {
@@ -149,10 +152,12 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
               }
             }
           } else if (eventType == XmlPullParser.TEXT) {
-            parent.addChild(TtmlNode.buildTextNode(xmlParser.getText()));
+            Assertions.checkNotNull(parent).addChild(TtmlNode.buildTextNode(xmlParser.getText()));
           } else if (eventType == XmlPullParser.END_TAG) {
             if (xmlParser.getName().equals(TtmlNode.TAG_TT)) {
-              ttmlSubtitle = new TtmlSubtitle(nodeStack.peek(), globalStyles, regionMap, imageMap);
+              ttmlSubtitle =
+                  new TtmlSubtitle(
+                      Assertions.checkNotNull(nodeStack.peek()), globalStyles, regionMap, imageMap);
             }
             nodeStack.pop();
           }
@@ -166,7 +171,11 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
         xmlParser.next();
         eventType = xmlParser.getEventType();
       }
-      return ttmlSubtitle;
+      if (ttmlSubtitle != null) {
+        return ttmlSubtitle;
+      } else {
+        throw new SubtitleDecoderException("No TTML subtitles found");
+      }
     } catch (XmlPullParserException xppe) {
       throw new SubtitleDecoderException("Unable to decode source", xppe);
     } catch (IOException e) {
@@ -221,8 +230,8 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
       return defaultValue;
     }
     try {
-      int columns = Integer.parseInt(cellResolutionMatcher.group(1));
-      int rows = Integer.parseInt(cellResolutionMatcher.group(2));
+      int columns = Integer.parseInt(Assertions.checkNotNull(cellResolutionMatcher.group(1)));
+      int rows = Integer.parseInt(Assertions.checkNotNull(cellResolutionMatcher.group(2)));
       if (columns == 0 || rows == 0) {
         throw new SubtitleDecoderException("Invalid cell resolution " + columns + " " + rows);
       }
@@ -233,7 +242,9 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
     }
   }
 
+  @Nullable
   private TtsExtent parseTtsExtent(XmlPullParser xmlParser) {
+    @Nullable
     String ttsExtent = XmlPullParserUtil.getAttributeValue(xmlParser, TtmlNode.ATTR_TTS_EXTENT);
     if (ttsExtent == null) {
       return null;
@@ -245,8 +256,8 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
       return null;
     }
     try {
-      int width = Integer.parseInt(extentMatcher.group(1));
-      int height = Integer.parseInt(extentMatcher.group(2));
+      int width = Integer.parseInt(Assertions.checkNotNull(extentMatcher.group(1)));
+      int height = Integer.parseInt(Assertions.checkNotNull(extentMatcher.group(2)));
       return new TtsExtent(width, height);
     } catch (NumberFormatException e) {
       Log.w(TAG, "Ignoring malformed tts extent: " + ttsExtent);
@@ -258,24 +269,26 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
       XmlPullParser xmlParser,
       Map<String, TtmlStyle> globalStyles,
       CellResolution cellResolution,
-      TtsExtent ttsExtent,
+      @Nullable TtsExtent ttsExtent,
       Map<String, TtmlRegion> globalRegions,
       Map<String, String> imageMap)
       throws IOException, XmlPullParserException {
     do {
       xmlParser.next();
       if (XmlPullParserUtil.isStartTag(xmlParser, TtmlNode.TAG_STYLE)) {
-        String parentStyleId = XmlPullParserUtil.getAttributeValue(xmlParser, ATTR_STYLE);
+        @Nullable String parentStyleId = XmlPullParserUtil.getAttributeValue(xmlParser, ATTR_STYLE);
         TtmlStyle style = parseStyleAttributes(xmlParser, new TtmlStyle());
         if (parentStyleId != null) {
           for (String id : parseStyleIds(parentStyleId)) {
             style.chain(globalStyles.get(id));
           }
         }
-        if (style.getId() != null) {
-          globalStyles.put(style.getId(), style);
+        String styleId = style.getId();
+        if (styleId != null) {
+          globalStyles.put(styleId, style);
         }
       } else if (XmlPullParserUtil.isStartTag(xmlParser, TtmlNode.TAG_REGION)) {
+        @Nullable
         TtmlRegion ttmlRegion = parseRegionAttributes(xmlParser, cellResolution, ttsExtent);
         if (ttmlRegion != null) {
           globalRegions.put(ttmlRegion.id, ttmlRegion);
@@ -292,7 +305,7 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
     do {
       xmlParser.next();
       if (XmlPullParserUtil.isStartTag(xmlParser, TtmlNode.TAG_IMAGE)) {
-        String id = XmlPullParserUtil.getAttributeValue(xmlParser, "id");
+        @Nullable String id = XmlPullParserUtil.getAttributeValue(xmlParser, "id");
         if (id != null) {
           String encodedBitmapData = xmlParser.nextText();
           imageMap.put(id, encodedBitmapData);
@@ -309,9 +322,10 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
    * fractions. In case of missing tts:extent the pixel defined regions can't be parsed, and null is
    * returned.
    */
+  @Nullable
   private TtmlRegion parseRegionAttributes(
-      XmlPullParser xmlParser, CellResolution cellResolution, TtsExtent ttsExtent) {
-    String regionId = XmlPullParserUtil.getAttributeValue(xmlParser, TtmlNode.ATTR_ID);
+      XmlPullParser xmlParser, CellResolution cellResolution, @Nullable TtsExtent ttsExtent) {
+    @Nullable String regionId = XmlPullParserUtil.getAttributeValue(xmlParser, TtmlNode.ATTR_ID);
     if (regionId == null) {
       return null;
     }
@@ -319,14 +333,16 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
     float position;
     float line;
 
+    @Nullable
     String regionOrigin = XmlPullParserUtil.getAttributeValue(xmlParser, TtmlNode.ATTR_TTS_ORIGIN);
     if (regionOrigin != null) {
       Matcher originPercentageMatcher = PERCENTAGE_COORDINATES.matcher(regionOrigin);
       Matcher originPixelMatcher = PIXEL_COORDINATES.matcher(regionOrigin);
       if (originPercentageMatcher.matches()) {
         try {
-          position = Float.parseFloat(originPercentageMatcher.group(1)) / 100f;
-          line = Float.parseFloat(originPercentageMatcher.group(2)) / 100f;
+          position =
+              Float.parseFloat(Assertions.checkNotNull(originPercentageMatcher.group(1))) / 100f;
+          line = Float.parseFloat(Assertions.checkNotNull(originPercentageMatcher.group(2))) / 100f;
         } catch (NumberFormatException e) {
           Log.w(TAG, "Ignoring region with malformed origin: " + regionOrigin);
           return null;
@@ -337,8 +353,8 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
           return null;
         }
         try {
-          int width = Integer.parseInt(originPixelMatcher.group(1));
-          int height = Integer.parseInt(originPixelMatcher.group(2));
+          int width = Integer.parseInt(Assertions.checkNotNull(originPixelMatcher.group(1)));
+          int height = Integer.parseInt(Assertions.checkNotNull(originPixelMatcher.group(2)));
           // Convert pixel values to fractions.
           position = width / (float) ttsExtent.width;
           line = height / (float) ttsExtent.height;
@@ -362,14 +378,17 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
 
     float width;
     float height;
+    @Nullable
     String regionExtent = XmlPullParserUtil.getAttributeValue(xmlParser, TtmlNode.ATTR_TTS_EXTENT);
     if (regionExtent != null) {
       Matcher extentPercentageMatcher = PERCENTAGE_COORDINATES.matcher(regionExtent);
       Matcher extentPixelMatcher = PIXEL_COORDINATES.matcher(regionExtent);
       if (extentPercentageMatcher.matches()) {
         try {
-          width = Float.parseFloat(extentPercentageMatcher.group(1)) / 100f;
-          height = Float.parseFloat(extentPercentageMatcher.group(2)) / 100f;
+          width =
+              Float.parseFloat(Assertions.checkNotNull(extentPercentageMatcher.group(1))) / 100f;
+          height =
+              Float.parseFloat(Assertions.checkNotNull(extentPercentageMatcher.group(2))) / 100f;
         } catch (NumberFormatException e) {
           Log.w(TAG, "Ignoring region with malformed extent: " + regionOrigin);
           return null;
@@ -380,8 +399,8 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
           return null;
         }
         try {
-          int extentWidth = Integer.parseInt(extentPixelMatcher.group(1));
-          int extentHeight = Integer.parseInt(extentPixelMatcher.group(2));
+          int extentWidth = Integer.parseInt(Assertions.checkNotNull(extentPixelMatcher.group(1)));
+          int extentHeight = Integer.parseInt(Assertions.checkNotNull(extentPixelMatcher.group(2)));
           // Convert pixel values to fractions.
           width = extentWidth / (float) ttsExtent.width;
           height = extentHeight / (float) ttsExtent.height;
@@ -404,8 +423,9 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
     }
 
     @Cue.AnchorType int lineAnchor = Cue.ANCHOR_TYPE_START;
-    String displayAlign = XmlPullParserUtil.getAttributeValue(xmlParser,
-        TtmlNode.ATTR_TTS_DISPLAY_ALIGN);
+    @Nullable
+    String displayAlign =
+        XmlPullParserUtil.getAttributeValue(xmlParser, TtmlNode.ATTR_TTS_DISPLAY_ALIGN);
     if (displayAlign != null) {
       switch (Util.toLowerInvariant(displayAlign)) {
         case "center":
@@ -440,7 +460,8 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
     return parentStyleIds.isEmpty() ? new String[0] : Util.split(parentStyleIds, "\\s+");
   }
 
-  private TtmlStyle parseStyleAttributes(XmlPullParser parser, TtmlStyle style) {
+  @PolyNull
+  private TtmlStyle parseStyleAttributes(XmlPullParser parser, @PolyNull TtmlStyle style) {
     int attributeCount = parser.getAttributeCount();
     for (int i = 0; i < attributeCount; i++) {
       String attributeValue = parser.getAttributeValue(i);
@@ -527,21 +548,24 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
     return style;
   }
 
-  private TtmlStyle createIfNull(TtmlStyle style) {
+  private TtmlStyle createIfNull(@Nullable TtmlStyle style) {
     return style == null ? new TtmlStyle() : style;
   }
 
-  private TtmlNode parseNode(XmlPullParser parser, TtmlNode parent,
-      Map<String, TtmlRegion> regionMap, FrameAndTickRate frameAndTickRate)
+  private TtmlNode parseNode(
+      XmlPullParser parser,
+      @Nullable TtmlNode parent,
+      Map<String, TtmlRegion> regionMap,
+      FrameAndTickRate frameAndTickRate)
       throws SubtitleDecoderException {
     long duration = C.TIME_UNSET;
     long startTime = C.TIME_UNSET;
     long endTime = C.TIME_UNSET;
     String regionId = TtmlNode.ANONYMOUS_REGION_ID;
-    String imageId = null;
-    String[] styleIds = null;
+    @Nullable String imageId = null;
+    @Nullable String[] styleIds = null;
     int attributeCount = parser.getAttributeCount();
-    TtmlStyle style = parseStyleAttributes(parser, null);
+    @Nullable TtmlStyle style = parseStyleAttributes(parser, null);
     for (int i = 0; i < attributeCount; i++) {
       String attr = parser.getAttributeName(i);
       String value = parser.getAttributeValue(i);
@@ -636,7 +660,7 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
     }
 
     if (matcher.matches()) {
-      String unit = matcher.group(3);
+      String unit = Assertions.checkNotNull(matcher.group(3));
       switch (unit) {
         case "px":
           out.setFontSizeUnit(TtmlStyle.FONT_SIZE_UNIT_PIXEL);
@@ -650,7 +674,7 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
         default:
           throw new SubtitleDecoderException("Invalid unit for fontSize: '" + unit + "'.");
       }
-      out.setFontSize(Float.valueOf(matcher.group(1)));
+      out.setFontSize(Float.parseFloat(Assertions.checkNotNull(matcher.group(1))));
     } else {
       throw new SubtitleDecoderException("Invalid expression for fontSize: '" + expression + "'.");
     }
@@ -671,18 +695,18 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
       throws SubtitleDecoderException {
     Matcher matcher = CLOCK_TIME.matcher(time);
     if (matcher.matches()) {
-      String hours = matcher.group(1);
+      String hours = Assertions.checkNotNull(matcher.group(1));
       double durationSeconds = Long.parseLong(hours) * 3600;
-      String minutes = matcher.group(2);
+      String minutes = Assertions.checkNotNull(matcher.group(2));
       durationSeconds += Long.parseLong(minutes) * 60;
-      String seconds = matcher.group(3);
+      String seconds = Assertions.checkNotNull(matcher.group(3));
       durationSeconds += Long.parseLong(seconds);
-      String fraction = matcher.group(4);
+      @Nullable String fraction = matcher.group(4);
       durationSeconds += (fraction != null) ? Double.parseDouble(fraction) : 0;
-      String frames = matcher.group(5);
+      @Nullable String frames = matcher.group(5);
       durationSeconds += (frames != null)
           ? Long.parseLong(frames) / frameAndTickRate.effectiveFrameRate : 0;
-      String subframes = matcher.group(6);
+      @Nullable String subframes = matcher.group(6);
       durationSeconds += (subframes != null)
           ? ((double) Long.parseLong(subframes)) / frameAndTickRate.subFrameRate
               / frameAndTickRate.effectiveFrameRate
@@ -691,9 +715,9 @@ public final class TtmlDecoder extends SimpleSubtitleDecoder {
     }
     matcher = OFFSET_TIME.matcher(time);
     if (matcher.matches()) {
-      String timeValue = matcher.group(1);
+      String timeValue = Assertions.checkNotNull(matcher.group(1));
       double offsetSeconds = Double.parseDouble(timeValue);
-      String unit = matcher.group(2);
+      String unit = Assertions.checkNotNull(matcher.group(2));
       switch (unit) {
         case "h":
           offsetSeconds *= 3600;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlNode.java b/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlNode.java
index b025a4e139..aee5b07632 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlNode.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlNode.java
@@ -31,6 +31,7 @@ import java.util.Map;
 import java.util.Map.Entry;
 import java.util.TreeMap;
 import java.util.TreeSet;
+import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
 
 /**
  * A package internal representation of TTML node.
@@ -93,7 +94,7 @@ import java.util.TreeSet;
   private final HashMap<String, Integer> nodeStartsByRegion;
   private final HashMap<String, Integer> nodeEndsByRegion;
 
-  private List<TtmlNode> children;
+  @MonotonicNonNull private List<TtmlNode> children;
 
   public static TtmlNode buildTextNode(String text) {
     return new TtmlNode(
@@ -196,6 +197,7 @@ import java.util.TreeSet;
     }
   }
 
+  @Nullable
   public String[] getStyleIds() {
     return styleIds;
   }
@@ -217,7 +219,7 @@ import java.util.TreeSet;
 
     // Create image based cues.
     for (Pair<String, String> regionImagePair : regionImageOutputs) {
-      String encodedBitmapData = imageMap.get(regionImagePair.second);
+      @Nullable String encodedBitmapData = imageMap.get(regionImagePair.second);
       if (encodedBitmapData == null) {
         // Image reference points to an invalid image. Do nothing.
         continue;
@@ -225,7 +227,7 @@ import java.util.TreeSet;
 
       byte[] bitmapData = Base64.decode(encodedBitmapData, Base64.DEFAULT);
       Bitmap bitmap = BitmapFactory.decodeByteArray(bitmapData, /* offset= */ 0, bitmapData.length);
-      TtmlRegion region = regionMap.get(regionImagePair.first);
+      TtmlRegion region = Assertions.checkNotNull(regionMap.get(regionImagePair.first));
 
       cues.add(
           new Cue.Builder()
@@ -241,7 +243,7 @@ import java.util.TreeSet;
 
     // Create text based cues.
     for (Entry<String, SpannableStringBuilder> entry : regionTextOutputs.entrySet()) {
-      TtmlRegion region = regionMap.get(entry.getKey());
+      TtmlRegion region = Assertions.checkNotNull(regionMap.get(entry.getKey()));
       cues.add(
           new Cue(
               cleanUpText(entry.getValue()),
@@ -286,7 +288,7 @@ import java.util.TreeSet;
     String resolvedRegionId = ANONYMOUS_REGION_ID.equals(regionId) ? inheritedRegion : regionId;
 
     if (isTextNode && descendsPNode) {
-      getRegionOutput(resolvedRegionId, regionOutputs).append(text);
+      getRegionOutput(resolvedRegionId, regionOutputs).append(Assertions.checkNotNull(text));
     } else if (TAG_BR.equals(tag) && descendsPNode) {
       getRegionOutput(resolvedRegionId, regionOutputs).append('\n');
     } else if (isActive(timeUs)) {
@@ -330,7 +332,7 @@ import java.util.TreeSet;
       int start = nodeStartsByRegion.containsKey(regionId) ? nodeStartsByRegion.get(regionId) : 0;
       int end = entry.getValue();
       if (start != end) {
-        SpannableStringBuilder regionOutput = regionOutputs.get(regionId);
+        SpannableStringBuilder regionOutput = Assertions.checkNotNull(regionOutputs.get(regionId));
         applyStyleToOutput(globalStyles, regionOutput, start, end);
       }
     }
@@ -344,7 +346,7 @@ import java.util.TreeSet;
       SpannableStringBuilder regionOutput,
       int start,
       int end) {
-    TtmlStyle resolvedStyle = TtmlRenderUtil.resolveStyle(style, styleIds, globalStyles);
+    @Nullable TtmlStyle resolvedStyle = TtmlRenderUtil.resolveStyle(style, styleIds, globalStyles);
     if (resolvedStyle != null) {
       TtmlRenderUtil.applyStylesToSpan(regionOutput, start, end, resolvedStyle);
     }
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlRenderUtil.java b/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlRenderUtil.java
index 25395431de..e1f3d5e887 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlRenderUtil.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlRenderUtil.java
@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.text.ttml;
 
+import android.text.Layout.Alignment;
 import android.text.SpannableStringBuilder;
 import android.text.Spanned;
 import android.text.style.AbsoluteSizeSpan;
@@ -26,6 +27,7 @@ import android.text.style.StrikethroughSpan;
 import android.text.style.StyleSpan;
 import android.text.style.TypefaceSpan;
 import android.text.style.UnderlineSpan;
+import androidx.annotation.Nullable;
 import com.google.android.exoplayer2.text.SpanUtil;
 import java.util.Map;
 
@@ -34,30 +36,35 @@ import java.util.Map;
  */
 /* package */ final class TtmlRenderUtil {
 
-  public static TtmlStyle resolveStyle(TtmlStyle style, String[] styleIds,
-      Map<String, TtmlStyle> globalStyles) {
-    if (style == null && styleIds == null) {
-      // No styles at all.
-      return null;
-    } else if (style == null && styleIds.length == 1) {
-      // Only one single referential style present.
-      return globalStyles.get(styleIds[0]);
-    } else if (style == null && styleIds.length > 1) {
-      // Only multiple referential styles present.
-      TtmlStyle chainedStyle = new TtmlStyle();
-      for (String id : styleIds) {
-        chainedStyle.chain(globalStyles.get(id));
+  @Nullable
+  public static TtmlStyle resolveStyle(
+      @Nullable TtmlStyle style, @Nullable String[] styleIds, Map<String, TtmlStyle> globalStyles) {
+    if (style == null) {
+      if (styleIds == null) {
+        // No styles at all.
+        return null;
+      } else if (styleIds.length == 1) {
+        // Only one single referential style present.
+        return globalStyles.get(styleIds[0]);
+      } else if (styleIds.length > 1) {
+        // Only multiple referential styles present.
+        TtmlStyle chainedStyle = new TtmlStyle();
+        for (String id : styleIds) {
+          chainedStyle.chain(globalStyles.get(id));
+        }
+        return chainedStyle;
       }
-      return chainedStyle;
-    } else if (style != null && styleIds != null && styleIds.length == 1) {
-      // Merge a single referential style into inline style.
-      return style.chain(globalStyles.get(styleIds[0]));
-    } else if (style != null && styleIds != null && styleIds.length > 1) {
-      // Merge multiple referential styles into inline style.
-      for (String id : styleIds) {
-        style.chain(globalStyles.get(id));
+    } else /* style != null */ {
+      if (styleIds != null && styleIds.length == 1) {
+        // Merge a single referential style into inline style.
+        return style.chain(globalStyles.get(styleIds[0]));
+      } else if (styleIds != null && styleIds.length > 1) {
+        // Merge multiple referential styles into inline style.
+        for (String id : styleIds) {
+          style.chain(globalStyles.get(id));
+        }
+        return style;
       }
-      return style;
     }
     // Only inline styles available.
     return style;
@@ -100,10 +107,11 @@ import java.util.Map;
           end,
           Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
     }
-    if (style.getTextAlign() != null) {
+    @Nullable Alignment textAlign = style.getTextAlign();
+    if (textAlign != null) {
       SpanUtil.addOrReplaceSpan(
           builder,
-          new AlignmentSpan.Standard(style.getTextAlign()),
+          new AlignmentSpan.Standard(textAlign),
           start,
           end,
           Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlStyle.java b/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlStyle.java
index 2ccbe7d85b..0c7c90afc5 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlStyle.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/text/ttml/TtmlStyle.java
@@ -18,9 +18,11 @@ package com.google.android.exoplayer2.text.ttml;
 import android.graphics.Typeface;
 import android.text.Layout;
 import androidx.annotation.IntDef;
+import androidx.annotation.Nullable;
 import java.lang.annotation.Documented;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
+import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
 
 /**
  * Style object of a <code>TtmlNode</code>
@@ -58,7 +60,7 @@ import java.lang.annotation.RetentionPolicy;
   private static final int OFF = 0;
   private static final int ON = 1;
 
-  private String fontFamily;
+  private @MonotonicNonNull String fontFamily;
   private int fontColor;
   private boolean hasFontColor;
   private int backgroundColor;
@@ -69,8 +71,8 @@ import java.lang.annotation.RetentionPolicy;
   @OptionalBoolean private int italic;
   @FontSizeUnit private int fontSizeUnit;
   private float fontSize;
-  private String id;
-  private Layout.Alignment textAlign;
+  private @MonotonicNonNull String id;
+  private Layout.@MonotonicNonNull Alignment textAlign;
 
   public TtmlStyle() {
     linethrough = UNSPECIFIED;
@@ -122,6 +124,7 @@ import java.lang.annotation.RetentionPolicy;
     return this;
   }
 
+  @Nullable
   public String getFontFamily() {
     return fontFamily;
   }
@@ -171,7 +174,7 @@ import java.lang.annotation.RetentionPolicy;
    *
    * @param ancestor the referential style to inherit from
    */
-  public TtmlStyle chain(TtmlStyle ancestor) {
+  public TtmlStyle chain(@Nullable TtmlStyle ancestor) {
     return inherit(ancestor, true);
   }
 
@@ -182,11 +185,11 @@ import java.lang.annotation.RetentionPolicy;
    *
    * @param ancestor the ancestor style to inherit from
    */
-  public TtmlStyle inherit(TtmlStyle ancestor) {
+  public TtmlStyle inherit(@Nullable TtmlStyle ancestor) {
     return inherit(ancestor, false);
   }
 
-  private TtmlStyle inherit(TtmlStyle ancestor, boolean chaining) {
+  private TtmlStyle inherit(@Nullable TtmlStyle ancestor, boolean chaining) {
     if (ancestor != null) {
       if (!hasFontColor && ancestor.hasFontColor) {
         setFontColor(ancestor.fontColor);
@@ -197,7 +200,7 @@ import java.lang.annotation.RetentionPolicy;
       if (italic == UNSPECIFIED) {
         italic = ancestor.italic;
       }
-      if (fontFamily == null) {
+      if (fontFamily == null && ancestor.fontFamily != null) {
         fontFamily = ancestor.fontFamily;
       }
       if (linethrough == UNSPECIFIED) {
@@ -206,7 +209,7 @@ import java.lang.annotation.RetentionPolicy;
       if (underline == UNSPECIFIED) {
         underline = ancestor.underline;
       }
-      if (textAlign == null) {
+      if (textAlign == null && ancestor.textAlign != null) {
         textAlign = ancestor.textAlign;
       }
       if (fontSizeUnit == UNSPECIFIED) {
@@ -226,10 +229,12 @@ import java.lang.annotation.RetentionPolicy;
     return this;
   }
 
+  @Nullable
   public String getId() {
     return id;
   }
 
+  @Nullable
   public Layout.Alignment getTextAlign() {
     return textAlign;
   }