diff --git a/components/style/properties/longhand/position.mako.rs b/components/style/properties/longhand/position.mako.rs
index f89bb95368f..818899ba7cb 100644
--- a/components/style/properties/longhand/position.mako.rs
+++ b/components/style/properties/longhand/position.mako.rs
@@ -164,7 +164,8 @@ ${helpers.predefined_type("order", "Integer", "0",
     // FIXME: Gecko doesn't support content value yet.
     //
     // FIXME(emilio): I suspect this property shouldn't allow quirks, and this
-    // was just a mistake.
+    // was just a mistake, it's kind of justificable to support it given the
+    // spec grammar is just `content | <width>`, but other browsers don't...
     ${helpers.predefined_type(
         "flex-basis",
         "MozLength",
diff --git a/components/style/values/computed/length.rs b/components/style/values/computed/length.rs
index 7dc4f6dc7a1..a9d211a235e 100644
--- a/components/style/values/computed/length.rs
+++ b/components/style/values/computed/length.rs
@@ -891,9 +891,15 @@ pub enum ExtremumLength {
 impl ExtremumLength {
     /// Returns whether this size keyword can be used for the given writing-mode
     /// and property.
+    ///
+    /// TODO: After these values are supported for both axes (and maybe
+    /// unprefixed, see bug 1322780) all this complexity can go away, and
+    /// everything can be derived (no need for uncacheable stuff).
     fn valid_for(&self, wm: WritingMode, longhand: LonghandId) -> bool {
         // We only make sense on the inline axis.
         match longhand {
+            // FIXME(emilio): The flex-basis thing is not quite clear...
+            LonghandId::FlexBasis |
             LonghandId::MinWidth |
             LonghandId::MaxWidth |
             LonghandId::Width => !wm.is_vertical(),
@@ -919,7 +925,9 @@ impl ExtremumLength {
     }
 }
 
-/// A value suitable for a `min-width`, `min-height`, `width` or `height` property.
+/// A value suitable for a `min-width`, `min-height`, `width` or `height`
+/// property.
+///
 /// See values/specified/length.rs for more details.
 #[allow(missing_docs)]
 #[cfg_attr(feature = "servo", derive(MallocSizeOf))]