[css-backgrounds-3] Defined border-width: thin/medium/thick precisely. w3c#7254

Author: tabatkins

css-backgrounds-3/Overview.bs

@@ -1405,12 +1405,9 @@ Where
 <pre class=prod><dfn><<line-width>></dfn> = <<length [0,&infin;]>> | thin | medium | thick</pre>
 
 <p dfn-type=value dfn-for="<line-width>, border-width, border-top-width, border-left-width, border-bottom-width, border-right-width, border">
-The lengths corresponding to <dfn>thin</dfn>, <dfn>medium</dfn>, and <dfn>thick</dfn>
-are not specified, but the values are constant throughout a
-<span>document</span> and thin &le; medium &le; thick. A UA could,
-e.g., make the thickness depend on the ''medium'' font size: one
-choice might be 1, 3 &amp; 5px when the ''medium'' font size is 17px or
-less. Negative values are invalid.
+Negative values are invalid.
+The <dfn>thin</dfn>, <dfn>medium</dfn>, and <dfn>thick</dfn> keywords
+are equivalent to ''1px'', ''3px'', and ''5px'', respectively.
 
 <p>The 'border-width' property is a shorthand property for setting
 'border-top-width', 'border-right-width', 'border-bottom-width', and 'border-left-width'
@@ -2610,6 +2607,8 @@ Changes since the 22 December 2020 Candidate Recommendation Snapshot</h3>
     when it doesn't generate boxes only applies to ''display: none'',
     not ''display: contents''.
     (<a href="https://github.com/w3c/csswg-drafts/issues/3779">Issue 3779</a>)
+  <li>Precisely defined the size of 'border-width' ''thin'', ''medium'', and ''thick''.
+    (<a href="https://github.com/w3c/csswg-drafts/issues/7254">Issue 7254</a>)
   <li>Minor editorial improvements.
 </ul>