Allow mathematical expressions in <img sizes>

Tests: web-platform-tests/wpt#10167.

Fixes whatwg#3003.

Author: musgravejw

source

@@ -3756,7 +3756,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li>The <dfn data-x-href="https://drafts.csswg.org/css-values/#in">'in'</dfn> unit</li>
      <li>The <dfn data-x-href="https://drafts.csswg.org/css-values/#px">'px'</dfn> unit</li>
      <li>The <dfn data-x-href="https://drafts.csswg.org/css-values/#funcdef-attr">'attr()'</dfn> function</li>
-     <li>The <dfn data-x-href="https://drafts.csswg.org/css-values/#funcdef-calc">'calc()'</dfn> function</li>
+     <li>The <dfn data-x-href="https://drafts.csswg.org/css-values/#math-function">math functions</dfn></li>
     </ul>
 
     <p>The term <dfn data-x="css-styling-attribute"
@@ -27060,7 +27060,7 @@ was an English &lt;a href="/wiki/Music_hall">music hall&lt;/a> singer, ...</pre>
 <dfn>&lt;source-size-value></dfn> = <span>&lt;length></span></pre>
 
   <p>A <span>&lt;source-size-value></span> must not be negative, and must not use CSS functions
-  other than the <span>'calc()'</span> function.</p>
+  other than the <span>math functions</span>.</p>
 
   <p>The <span>&lt;source-size-value></span> gives the intended layout width of the image. The
   author can specify different widths for different environments with
@@ -28236,7 +28236,7 @@ was an English &lt;a href="/wiki/Music_hall">music hall&lt;/a> singer, ...</pre>
    is a valid non-negative <span>&lt;source-size-value></span>,
    let <var>size</var> be its value
    and remove the <span>component value</span> from <var>unparsed size</var>.
-   Any CSS function other than the <span>'calc()'</span> function is invalid.
+   Any CSS function other than the <span>math functions</span> is invalid.
    Otherwise, there is a <span data-x="concept-microsyntax-parse-error">parse error</span>;
    continue to the next iteration of this algorithm.</p></li>