[css-syntax] Fix up some markup and add definitions for 'invalid' and 'ignore'

Author: tabatkins

css-syntax/Overview.bs

@@ -45,15 +45,15 @@ Description of CSS's Syntax</h2>
 
 	<em>This section is not normative.</em>
 
-	A CSS document is a series of <a>qualified rules</a>,
-	which are usually style rules that apply CSS properties to elements,
-	and <a>at-rules</a>,
+	A CSS document is a series of <a>style rules</a>--
+	which are <a>qualified rules</a> that apply styles to elements in a document--
+	and <a>at-rules</a>--
 	which define special processing rules or values for the CSS document.
 
-	A qualified rule starts with a prelude
+	A <a>qualified rule</a> starts with a prelude
 	then has a {}-wrapped block containing a sequence of declarations.
-	The meaning of the prelude varies based on the context that the rule appears in -
-	for style rules, it's a selector which specifies what elements the declarations will apply to.
+	The meaning of the prelude varies based on the context that the rule appears in--
+	for <a>style rules</a>, it's a selector which specifies what elements the declarations will apply to.
 	Each declaration has a name,
 	followed by a colon and the declaration value.
 	Declarations are separated by semicolons.
@@ -79,8 +79,8 @@ Description of CSS's Syntax</h2>
 		Similarly, their 'text-decoration' property should have the value ''underline''.
 	</div>
 
-	At-rules are all different, but they have a basic structure in common.
-	They start with an "@" <a>code point</a> followed by their name.
+	<a>At-rules</a> are all different, but they have a basic structure in common.
+	They start with an "@" <a>code point</a> followed by their name as a CSS keyword.
 	Some <a>at-rules</a> are simple statements,
 	with their name followed by more CSS values to specify their behavior,
 	and finally ended by a semicolon.
@@ -174,13 +174,13 @@ Error Handling</h3>
 	the parser attempts to recover gracefully,
 	throwing away only the minimum amount of content
 	before returning to parsing as normal.
-	This is because errors aren't always mistakes -
+	This is because errors aren't always mistakes--
 	new syntax looks like an error to an old parser,
 	and it's useful to be able to add new syntax to the language
 	without worrying about stylesheets that include it being completely broken in older UAs.
 
 	The precise error-recovery behavior is detailed in the parser itself,
-	but it's simple enough that a short description is fairly accurate:
+	but it's simple enough that a short description is fairly accurate.
 
 	<ul>
 		<li>
@@ -189,7 +189,7 @@ Error Handling</h3>
 			Anything else starts a qualified rule,
 			and is included in the rule's prelude.
 			This may produce an invalid selector,
-			but that's not the concern of the CSS parser &mdash;
+			but that's not the concern of the CSS parser--
 			at worst, it means the selector will match nothing.
 
 		<li>
@@ -224,6 +224,13 @@ Error Handling</h3>
 			and thus thrown away when they are verified against their grammar.
 	</ul>
 
+	After each construct (declaration, style rule, at-rule) is parsed,
+	the user agent checks it against its expected grammar.
+	If it does not match the grammar,
+	it's <dfn export for=css>invalid</dfn>,
+	and gets <dfn export for=css>ignored</dfn> by the UA,
+	which treats it as if it wasn't there at all.
+
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@@ -3189,7 +3196,7 @@ CSS stylesheets</h2>
 	first <i>parse a stylesheet</i>.
 	Interpret all of the resulting top-level <i>qualified rules</i> as <i>style rules</i>, defined below.
 
-	If any style rule is invalid,
+	If any style rule is <a>invalid</a>,
 	or any at-rule is not recognized or is invalid according to its grammar or context,
 	it's a <i>parse error</i>.
 	Discard that rule.
@@ -3207,15 +3214,15 @@ Style rules</h3>
 	The prelude of the qualified rule is parsed as a
 	<a href="http://dev.w3.org/csswg/selectors4/#selector-list">selector list</a>.
 	If this results in an <a href="http://dev.w3.org/csswg/selectors4/#invalid">invalid selector list</a>,
-	the entire style rule is invalid.
+	the entire style rule is <a>invalid</a>.
 
 	The content of the qualified rule’s block is parsed as a
 	<a lt="parse a list of declarations">list of declarations</a>.
 	Unless defined otherwise by another specification or a future level of this specification,
-	at-rules in that list are invalid
+	at-rules in that list are <a>invalid</a>
 	and must be ignored.
 	Declaration for an unknown CSS property
-	or whose value does not match the syntax defined by the property are invalid
+	or whose value does not match the syntax defined by the property are <a>invalid</a>
 	and must be ignored.
 	The validity of the style rule’s contents have no effect on the validity of the style rule itself.
 	Unless otherwise specified, property names are <a>ASCII case-insensitive</a>.