Explain that http-equiv has nothing to do with HTTP headers

Closes #7435. See also #2335 which discusses name="" vs. http-equiv="".

Author: domenic

source

@@ -17198,6 +17198,28 @@ interface <dfn interface>HTMLMetaElement</dfn> : <span>HTMLElement</span> {
   data-x="attr-meta-http-equiv">http-equiv</code></dfn> attribute is specified on a
   <code>meta</code> element, the element is a pragma directive.</p>
 
+  <div class="warning" id="warning-http-equiv-unrelated-to-http-headers">
+   <p>Despite the name <code data-x="attr-meta-http-equiv">http-equiv</code>, pragma directives are
+   almost entirely unrelated to HTTP headers. Implementers and web developers are best off thinking
+   of them as entirely separate, and the name as being a historical accident.</p>
+
+   <p>In more detail, although the <code data-x="attr-meta-http-equiv-refresh">refresh</code>
+   keyword has the same processing model as the corresponding `<code>Refresh</code>` header, every
+   other standardized pragma directive has at least slightly different behavior than the
+   similarly-named header. (And usually dramatically-different behavior.)</p>
+
+   <p>Implementers or specification writers contemplating adding new document-level pragmas or HTTP
+   header-controlled switches should be cautious about this mismatch, and avoid perpetuating the
+   existing confusion by adding the same or similar behavior to both an HTTP header and an <code
+   data-x="attr-meta-http-equiv">http-equiv</code> pragma. Instead, consider providing only an HTTP
+   header, or if an in-document pragma is needed, consider adding a new attribute to
+   <code>meta</code> similar to the model used by the <code
+   data-x="attr-meta-charset">charset</code> attribute. (Note that avoiding in-document pragmas is
+   often the better choice, since the DOM is mutable. Thus, even in the simple case where the
+   developer does not add, remove, or mutate <code>meta</code> elements, the policy will go from
+   un-applied to applied during parsing, which can have complex implications.)</p>
+  </div>
+
   <p>The <code data-x="attr-meta-http-equiv">http-equiv</code> attribute is an <span>enumerated
   attribute</span> with the following keywords and states:</p>