[css-values] Better wording for base url stuff.

Author: fantasai

css-values/Overview.html

@@ -797,9 +797,8 @@ <h3 id=urls><span class=secno>3.4. </span> Resource Locators: the ‘<a
    rel=biblioentry>[URI]<!--{{!URI}}--></a>) are resolved to full URIs using
    a base URI. RFC 3986, section 3, defines the normative algorithm for
    this process. For CSS style sheets, the base URI is that of the style
-   sheet itself, which is not necessarily that of the source document. Style
-   sheets embedded within a document must use the base URL associated with
-   their container.
+   sheet itself, not that of the source document. Style sheets embedded
+   within a document have the base URL associated with their container.
 
   <p>When a <a href="#url-value"><i>&lt;url></i></a> appears in the computed
    value of a property, it is resolved to an absolute URL, as described in

css-values/Overview.src.html

@@ -462,8 +462,8 @@ <h3 id="urls">
 	using a base URI. RFC&nbsp;3986, section&nbsp;3, defines the normative
 	algorithm for this process.
 	For CSS style sheets, the base URI is that of the style sheet itself,
-	which is not necessarily that of the source document.
-	Style sheets embedded within a document must use
+	not that of the source document.
+	Style sheets embedded within a document have
 	the base URL associated with their container.
 
 	<p>When a <i>&lt;url></i> appears in the computed value of a property,