diff --git a/app-common/src/main/kotlin/net/thunderbird/android/feature/mail/message/reader/api/css/DefaultCssClassNameProvider.kt b/app-common/src/main/kotlin/net/thunderbird/android/feature/mail/message/reader/api/css/DefaultCssClassNameProvider.kt
index 45c41efe0cc..6b51bb48dec 100644
--- a/app-common/src/main/kotlin/net/thunderbird/android/feature/mail/message/reader/api/css/DefaultCssClassNameProvider.kt
+++ b/app-common/src/main/kotlin/net/thunderbird/android/feature/mail/message/reader/api/css/DefaultCssClassNameProvider.kt
@@ -15,12 +15,14 @@ class DefaultCssClassNameProvider(
         if (featureFlagProvider.provide(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles).isEnabled()) {
             "${defaultNamespaceClassName}__plain-text-message-pre"
         } else {
+            // TODO(#10498): Remove when UseNewMessageReaderCssStyles is no longer required
             EmailTextToHtml.K9MAIL_CSS_CLASS
         }
     override val signatureClassName: String =
         if (featureFlagProvider.provide(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles).isEnabled()) {
             "${defaultNamespaceClassName}__signature"
         } else {
+            // TODO(#10498): Remove when UseNewMessageReaderCssStyles is no longer required
             "k9mail-signature"
         }
 }
diff --git a/app-k9mail/src/debug/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt b/app-k9mail/src/debug/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt
index d378fbf238d..1df2b31bfa7 100644
--- a/app-k9mail/src/debug/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt
+++ b/app-k9mail/src/debug/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt
@@ -23,6 +23,7 @@ class K9FeatureFlagFactory : FeatureFlagFactory {
                 FeatureFlag(MessageListFeatureFlags.UseComposeForMessageListItems, enabled = false),
                 FeatureFlag(MessageViewFeatureFlags.ActionExportEml, enabled = true),
                 FeatureFlag(AccountSettingsFeatureFlags.EnableAvatarCustomization, enabled = true),
+                // TODO(#10498): Remove when UseNewMessageReaderCssStyles is no longer required
                 FeatureFlag(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles, enabled = true),
                 FeatureFlag(MessageListFeatureFlags.EnableMessageListNewState, enabled = true),
             ),
diff --git a/app-k9mail/src/release/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt b/app-k9mail/src/release/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt
index 53bf2f77a61..b501dee7ac5 100644
--- a/app-k9mail/src/release/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt
+++ b/app-k9mail/src/release/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt
@@ -26,7 +26,9 @@ class K9FeatureFlagFactory : FeatureFlagFactory {
                 FeatureFlag(MessageListFeatureFlags.UseComposeForMessageListItems, enabled = false),
                 FeatureFlag(MessageViewFeatureFlags.ActionExportEml, enabled = false),
                 FeatureFlag(AccountSettingsFeatureFlags.EnableAvatarCustomization, enabled = true),
-                FeatureFlag(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles, enabled = false),
+                // TODO(#10498): Clean up all the legacy code that is wrapped with UseNewMessageReaderCssStyles
+                //  once it no longer required
+                FeatureFlag(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles, enabled = true),
                 FeatureFlag(MessageListFeatureFlags.EnableMessageListNewState, enabled = false),
             ),
         )
diff --git a/app-thunderbird/src/debug/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt b/app-thunderbird/src/debug/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
index bbcb8dba843..7ff02280e44 100644
--- a/app-thunderbird/src/debug/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
+++ b/app-thunderbird/src/debug/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
@@ -26,6 +26,7 @@ class TbFeatureFlagFactory : FeatureFlagFactory {
                 FeatureFlag(MessageListFeatureFlags.UseComposeForMessageListItems, enabled = false),
                 FeatureFlag(MessageViewFeatureFlags.ActionExportEml, enabled = true),
                 FeatureFlag(AccountSettingsFeatureFlags.EnableAvatarCustomization, enabled = true),
+                // TODO(#10498): Remove when UseNewMessageReaderCssStyles is no longer required
                 FeatureFlag(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles, enabled = true),
                 FeatureFlag(MessageListFeatureFlags.EnableMessageListNewState, enabled = true),
             ),
diff --git a/app-thunderbird/src/release/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt b/app-thunderbird/src/release/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
index 00639d394bf..fb2200c89ad 100644
--- a/app-thunderbird/src/release/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
+++ b/app-thunderbird/src/release/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
@@ -26,7 +26,9 @@ class TbFeatureFlagFactory : FeatureFlagFactory {
                 FeatureFlag(MessageListFeatureFlags.UseComposeForMessageListItems, enabled = false),
                 FeatureFlag(MessageViewFeatureFlags.ActionExportEml, enabled = false),
                 FeatureFlag(AccountSettingsFeatureFlags.EnableAvatarCustomization, enabled = true),
-                FeatureFlag(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles, enabled = false),
+                // TODO(#10498): Clean up all the legacy code that is wrapped with UseNewMessageReaderCssStyles
+                //  once it no longer required
+                FeatureFlag(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles, enabled = true),
                 FeatureFlag(MessageListFeatureFlags.EnableMessageListNewState, enabled = false),
             ),
         )
diff --git a/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/MessageReaderFeatureFlags.kt b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/MessageReaderFeatureFlags.kt
index e093461c43d..1277f1a7a32 100644
--- a/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/MessageReaderFeatureFlags.kt
+++ b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/MessageReaderFeatureFlags.kt
@@ -3,5 +3,6 @@ package net.thunderbird.feature.mail.message.reader.api
 import net.thunderbird.core.featureflag.FeatureFlagKey
 
 object MessageReaderFeatureFlags {
+    // TODO(#10498): Remove when UseNewMessageReaderCssStyles is no longer required
     val UseNewMessageReaderCssStyles = FeatureFlagKey("use_new_message_reader_css_styles")
 }
diff --git a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultGlobalCssStyleProvider.kt b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultGlobalCssStyleProvider.kt
index 49de988d992..d7352266eff 100644
--- a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultGlobalCssStyleProvider.kt
+++ b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultGlobalCssStyleProvider.kt
@@ -54,6 +54,7 @@ internal class DefaultGlobalCssStyleProvider private constructor(
     }
 }
 
+// TODO(#10498): Remove when UseNewMessageReaderCssStyles is no longer required
 internal class LegacyGlobalCssStyleProvider(useDarkMode: Boolean) : GlobalCssStyleProvider {
     @Language("HTML")
     override val style: String = if (useDarkMode) {
diff --git a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/inject/FeatureMessageReaderModule.kt b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/inject/FeatureMessageReaderModule.kt
index 10d7044e788..4a48956461c 100644
--- a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/inject/FeatureMessageReaderModule.kt
+++ b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/inject/FeatureMessageReaderModule.kt
@@ -27,6 +27,7 @@ val featureMessageReaderModule = module {
                 cssVariableNameProvider = get(),
             )
         } else {
+            // TODO(#10498): Remove when UseNewMessageReaderCssStyles is no longer required
             LegacyGlobalCssStyleProvider.Factory()
         }
     }
diff --git a/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlProcessorFactory.kt b/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlProcessorFactory.kt
index fdd03b2ab8e..62903d93aa3 100644
--- a/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlProcessorFactory.kt
+++ b/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlProcessorFactory.kt
@@ -17,6 +17,7 @@ class HtmlProcessorFactory(
             if (featureFlagProvider.provide(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles).isEnabled()) {
                 setOf(cssClassNameProvider.rootClassName, cssClassNameProvider.mainContentClassName)
             } else {
+                // TODO(#10498): Remove when UseNewMessageReaderCssStyles is no longer required
                 emptySet()
             }
         return HtmlProcessor(customClasses, displayHtml)