fix broken links to CSS 2.1 stacking context

Author: svgeesus

compositing-1/Overview.bs

@@ -45,7 +45,7 @@ those properties are applied; these effects are applied after elements have been
 to the <a href="https://www.w3.org/TR/CSS2/visuren.html" title="Visual formatting model">Visual formatting model</a>
 from [[!CSS21]]. Some values of these properties result in the creation of a
 <a href="https://www.w3.org/TR/CSS2/visuren.html#containing-block" title="Visual formatting model">containing block</a>,
-and/or the creation of a <a spec="css21">stacking context</a>.
+and/or the creation of a <a href="https://www.w3.org/tr/css21/visuren.html#x43">stacking context</a>.
 
 The 'background-blend-mode' property also builds upon the properties defined in the <a href="https://www.w3.org/TR/css3-background/#placement" title="Backgrounds and Borders">CSS  Backgrounds and Borders</a> module [[!CSS3BG]].
 
@@ -76,11 +76,11 @@ The compositing model must follow the <a href="https://www.w3.org/TR/SVG11/rende
 <h3 id="csscompositingrules_CSS">Behavior specific to HTML</h3>
 <!-- <p>
 If an element specifies non-default blending or <a href="https://www.w3.org/TR/css3-color/#transparency">'opacity'</a>, its <a href="https://www.w3.org/TR/css3-2d-transforms/#transform-style-property">transform-style</a> [[CSS3-TRANSFORMS]] and that of all of its children must revert to 'flat'.<br>
-The application of blending other than 'normal' to an element must also establish a <a spec="css21">stacking context</a> [[!CSS21]] the same way that CSS <a href="https://www.w3.org/TR/css3-color/#transparency">'opacity'</a> does. One of the consequences is that elements with z-index must not honor the depth of elements outside of the group.<br> -->
+The application of blending other than 'normal' to an element must also establish a <a href="https://www.w3.org/tr/css21/visuren.html#x43">stacking context</a> [[!CSS21]] the same way that CSS <a href="https://www.w3.org/TR/css3-color/#transparency">'opacity'</a> does. One of the consequences is that elements with z-index must not honor the depth of elements outside of the group.<br> -->
 
 Everything in CSS that creates a <a href="https://www.w3.org/TR/CSS21/zindex.html">stacking context</a> must be considered an <a href="#isolatedgroups">‘isolated’ group</a>. HTML elements themselves should not create groups.
 
-An element that has blending applied, must blend with all the underlying content of the <a spec="css21">stacking context</a> [[!CSS21]] that that element belongs to.
+An element that has blending applied, must blend with all the underlying content of the <a href="https://www.w3.org/tr/css21/visuren.html#x43">stacking context</a> [[!CSS21]] that that element belongs to.
 
 The root element for an HTML document is the <a href="https://dom.spec.whatwg.org/#document-element">document element</a>.
 
@@ -133,7 +133,7 @@ The syntax of the property of <<blend-mode>> is given with:
     <a value>hue</a> | <a value>saturation</a> | <a value>color</a> | <a value>luminosity</a>
 </pre>
 
-Note: Applying a blendmode other than <a value>normal</a> to the element must establish a new <a spec="css21">stacking context</a> [[!CSS21]]. This group must then be blended and composited with the <a spec="css21">stacking context</a> that contains the element.
+Note: Applying a blendmode other than <a value>normal</a> to the element must establish a new <a href="https://www.w3.org/tr/css21/visuren.html#x43">stacking context</a> [[!CSS21]]. This group must then be blended and composited with the <a href="https://www.w3.org/tr/css21/visuren.html#x43">stacking context</a> that contains the element.
 
 <div class="example">
 <p>Given the following sample markup:</p>