diff --git a/html5ever/src/tree_builder/mod.rs b/html5ever/src/tree_builder/mod.rs
index c432e58d..5eeee1f3 100644
--- a/html5ever/src/tree_builder/mod.rs
+++ b/html5ever/src/tree_builder/mod.rs
@@ -46,21 +46,24 @@ mod types;
 #[derive(Copy, Clone)]
 pub struct TreeBuilderOpts {
     /// Report all parse errors described in the spec, at some
-    /// performance penalty?  Default: false
+    /// performance penalty? Default: false
     pub exact_errors: bool,
 
     /// Is scripting enabled?
+    ///
+    /// This affects how `<noscript>` elements are parsed:
+    ///   - If scripting **is** enabled then the contents of a `<noscript>` element are parsed as a single text node
+    ///   - If scriping is **not** enabled then the contents of a `<noscript>` element are parsed as a normal tree of nodes
     pub scripting_enabled: bool,
 
-    /// Is this an `iframe srcdoc` document?
+    /// Is this document being parsed from the `srcdoc` attribute of an `<iframe>` element?
+    ///
+    /// This affects heuristics that infer `QuirksMode` from `<!DOCTYPE>`.
     pub iframe_srcdoc: bool,
 
     /// Should we drop the DOCTYPE (if any) from the tree?
     pub drop_doctype: bool,
 
-    /// Obsolete, ignored.
-    pub ignore_missing_rules: bool,
-
     /// Initial TreeBuilder quirks mode. Default: NoQuirks
     pub quirks_mode: QuirksMode,
 }
@@ -72,7 +75,6 @@ impl Default for TreeBuilderOpts {
             scripting_enabled: true,
             iframe_srcdoc: false,
             drop_doctype: false,
-            ignore_missing_rules: false,
             quirks_mode: NoQuirks,
         }
     }