treat Last-Event-ID and Client-Hints headers as simple headers

- Last-Event-ID is defined in the HTML spec as a utf-8 string, hence we
  don't place any restrictions on its value
- Each Client Hints header has a BNF grammar that should be validated by
  the user agent; the header is defined as simple if the name of the
  header matches one of CH header names, and its value conforms to the
  defined grammar

Closes:
- whatwg#52
- httpwg/http-extensions#141

Author: igrigorik

Overview.src.html

@@ -336,13 +336,23 @@ <h4 id=terminology-headers>Headers</h4>
 <hr>
 
 <p>A <dfn>simple header</dfn> is a <span title=concept-header>header</span> whose
-<span title=concept-header-name>name</span> is `<code title>Accept</code>`,
-`<code title>Accept-Language</code>`, or `<code title>Content-Language</code>`, or whose
-<span title=concept-header-name>name</span> is `<code title>Content-Type</code>` and
-<span title=concept-header-value>value</span>, <span title=concept-header-parse>once parsed</span>,
-has a MIME type (ignoring parameters) that is
-`<code title>application/x-www-form-urlencoded</code>`, `<code title>multipart/form-data</code>`, or
-`<code title>text/plain</code>`.
+<span title=concept-header-name>name</span> is one of
+
+<ul class=brief>
+  <li>`<code title>Accept</code>`
+  <li>`<code title>Accept-Language</code>`
+  <li>`<code title>Content-Language</code>`
+  <li>`<code title>Content-Type</code>` and whose <span title=concept-header-value>value</span>,
+  <span title=concept-header-parse>once parsed</span>, has a MIME type (ignoring parameters)
+  that is `<code title>application/x-www-form-urlencoded</code>`,
+  `<code title>multipart/form-data</code>`, or `<code title>text/plain</code>`
+  <li>`<code title><a href=https://html.spec.whatwg.org/multipage/comms.html#last-event-id>Last-Event-ID</a></code>`
+  <li>`<code title><a href=http://httpwg.org/http-extensions/client-hints.html#the-dpr-client-hint>DPR</a></code>`,
+  `<code title><a href=http://httpwg.org/http-extensions/client-hints.html#the-downlink-client-hint>Downlink</a></code>`,
+  `<code title><a href=http://httpwg.org/http-extensions/client-hints.html#the-save-data-hint>Save-Data</a></code>`,
+  `<code title><a href=http://httpwg.org/http-extensions/client-hints.html#the-viewport-width-client-hint>Viewport-Width</a></code>`, or
+  `<code title><a href=http://httpwg.org/http-extensions/client-hints.html#the-width-client-hint>Width</a></code>`, and whose value, once parsed, is not a failure.
+</ul>
 <!-- XXX * needs better xref
          * ignoring parameters has been the standard for a long time now
          * interesting test: "Content-Type: text/plain;" -->