diff --git a/library/ui/src/main/java/com/google/android/exoplayer2/ui/SpannedToHtmlConverter.java b/library/ui/src/main/java/com/google/android/exoplayer2/ui/SpannedToHtmlConverter.java
index e06edd8ded..496598bead 100644
--- a/library/ui/src/main/java/com/google/android/exoplayer2/ui/SpannedToHtmlConverter.java
+++ b/library/ui/src/main/java/com/google/android/exoplayer2/ui/SpannedToHtmlConverter.java
@@ -22,6 +22,7 @@ import android.text.Spanned;
 import android.text.style.AbsoluteSizeSpan;
 import android.text.style.BackgroundColorSpan;
 import android.text.style.ForegroundColorSpan;
+import android.text.style.RelativeSizeSpan;
 import android.text.style.StyleSpan;
 import android.text.style.TypefaceSpan;
 import android.text.style.UnderlineSpan;
@@ -145,6 +146,9 @@ import java.util.regex.Pattern;
               ? absoluteSizeSpan.getSize()
               : absoluteSizeSpan.getSize() / displayDensity;
       return Util.formatInvariant("<span style='font-size:%.2fpx;'>", sizeCssPx);
+    } else if (span instanceof RelativeSizeSpan) {
+      return Util.formatInvariant(
+          "<span style='font-size:%.2f%%;'>", ((RelativeSizeSpan) span).getSizeChange());
     } else if (span instanceof TypefaceSpan) {
       @Nullable String fontFamily = ((TypefaceSpan) span).getFamily();
       return fontFamily != null
@@ -185,7 +189,8 @@ import java.util.regex.Pattern;
     if (span instanceof ForegroundColorSpan
         || span instanceof BackgroundColorSpan
         || span instanceof HorizontalTextInVerticalContextSpan
-        || span instanceof AbsoluteSizeSpan) {
+        || span instanceof AbsoluteSizeSpan
+        || span instanceof RelativeSizeSpan) {
       return "</span>";
     } else if (span instanceof TypefaceSpan) {
       @Nullable String fontFamily = ((TypefaceSpan) span).getFamily();
diff --git a/library/ui/src/test/java/com/google/android/exoplayer2/ui/SpannedToHtmlConverterTest.java b/library/ui/src/test/java/com/google/android/exoplayer2/ui/SpannedToHtmlConverterTest.java
index c64d5cabcf..fc4d044a53 100644
--- a/library/ui/src/test/java/com/google/android/exoplayer2/ui/SpannedToHtmlConverterTest.java
+++ b/library/ui/src/test/java/com/google/android/exoplayer2/ui/SpannedToHtmlConverterTest.java
@@ -25,6 +25,7 @@ import android.text.Spanned;
 import android.text.style.AbsoluteSizeSpan;
 import android.text.style.BackgroundColorSpan;
 import android.text.style.ForegroundColorSpan;
+import android.text.style.RelativeSizeSpan;
 import android.text.style.StyleSpan;
 import android.text.style.TypefaceSpan;
 import android.text.style.UnderlineSpan;
@@ -130,6 +131,20 @@ public class SpannedToHtmlConverterTest {
     assertThat(html).isEqualTo("String with <span style='font-size:10.00px;'>10dp</span> section");
   }
 
+  @Test
+  public void convert_supportsRelativeSizeSpan() {
+    SpannableString spanned = new SpannableString("String with 10% section");
+    spanned.setSpan(
+        new RelativeSizeSpan(/* proportion= */ 10),
+        "String with ".length(),
+        "String with 10%".length(),
+        Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
+
+    String html = SpannedToHtmlConverter.convert(spanned, displayDensity);
+
+    assertThat(html).isEqualTo("String with <span style='font-size:10.00%;'>10%</span> section");
+  }
+
   @Test
   public void convert_supportsTypefaceSpan() {
     SpannableString spanned = new SpannableString("String with Times New Roman section");