Encourage web browser User-Agent value to start with Mozilla/5.0 (

This matches what all major web browsers already do and navigator.appVersion relies on this to a fault. See whatwg/html#11630 for context.

Author: annevk

fetch.bs

@@ -1419,6 +1419,10 @@ downloads. This format of range header value can be set using <a>add a range hea
 <a>implementation-defined</a> <a for=/>header value</a> for the `<code>User-Agent</code>`
 <a for=/>header</a>.
 
+<p class=note>For unfortunate web compatibility reasons, web browsers are strongly encouraged to
+have this value start with `<code>Mozilla/5.0 (</code>` and be generally modeled after other web
+browsers.
+
 <p>The <dfn>document `<code>Accept</code>` header value</dfn> is
 `<code>text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8</code>`.