semantics: remove rel-external, rel-pingback, and rel-sidebar as they…

… were gone from the spec.

semantics.html

@@ -231,8 +231,6 @@ <h3 id=new-relations>Other Link Relations in HTML5</h3>
 
 <p><code>rel="author"</code> is used to link to information about the author of the page. This can be a <code>mailto:</code> address, though it doesn&#8217;t have to be. It could simply link to a contact form or &#8220;about the author&#8221; page.
 
-<p><a href=http://www.whatwg.org/specs/web-apps/current-work/multipage/links.html#link-type-external>rel="external"</a> &#8220;indicates that the link is leading to a document that is not part of the site that the current document forms a part of.&#8221; I believe it was first popularized by <a href=http://www.wordpress.org/>WordPress</a>, which uses it on links left by commenters.
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 <p class=ss style="width:313px"><img src=i/openclipart.org_johnny_automatic_girl_feeding_birds.png width=313 height=384 alt="girl feeding birds">
 
 <p>HTML 4 defined <a href=http://www.w3.org/TR/html401/types.html#type-links><code>rel="start"</code>, <code>rel="prev"</code>, and <code>rel="next"</code></a> to define relations between pages that are part of a series (like chapters of a book, or even posts on a blog). The only one that was ever used correctly was <code>rel="next"</code>. People used <code>rel="previous"</code> instead of <code>rel="prev"</code>; they used <code>rel="begin"</code> and <code>rel="first"</code> instead of <code>rel="start"</code>; they used <code>rel="end"</code> instead of <code>rel="last"</code>. Oh, and &mdash; all by themselves &mdash; they made up <code>rel="up"</code> to point to a &#8220;parent&#8221; page.
@@ -255,16 +253,12 @@ <h3 id=new-relations>Other Link Relations in HTML5</h3>
 
 <p><a href=http://www.whatwg.org/specs/web-apps/current-work/multipage/links.html#link-type-noreferrer>rel="noreferrer"</a> &#8220;indicates that no referrer information is to be leaked when following the link.&#8221; No shipping browser currently supports this, but support <a href=http://webkit.org/blog/907/webkit-nightlies-support-html5-noreferrer-link-relation/>was recently added to WebKit nightlies</a>, so it will eventually be showing up in Safari, Google Chrome, and other WebKit-based browsers. [<a href=http://wearehugh.com/public/2009/04/rel-noreferrer.html>rel="noreferrer" test case</a>]
 
-<p><a href=http://www.whatwg.org/specs/web-apps/current-work/multipage/links.html#link-type-pingback>rel="pingback"</a> specifies the address of a &#8220;pingback&#8221; server. As explained in <a href=http://hixie.ch/specs/pingback/pingback-1.0>the Pingback specification</a>, &#8220;The pingback system is a way for a blog to be automatically notified when other Web sites link to it. ... It enables reverse linking &mdash; a way of going back up a chain of links rather than merely drilling down.&#8221; Blogging systems, notably WordPress, implement the pingback mechanism to notify authors that you have linked to them when creating a new blog post.
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 <p class=ss style="float:left;margin:0 1.75em 1.75em 0;width:271px"><img src=i/openclipart.org_johnny_automatic_dog_on_chair.png width=271 height=309 alt="dog on chair">
 
 <p><a href=http://www.whatwg.org/specs/web-apps/current-work/multipage/links.html#link-type-prefetch>rel="prefetch"</a> &#8220;indicates that preemptively fetching and caching the specified resource is likely to be beneficial, as it is highly likely that the user will require this resource.&#8221; Search engines sometimes add <code>&lt;link rel="prefetch" href="<i>URL of top search result</i>"></code> to the search results page if they feel that the top result is wildly more popular than any other. For example: using Firefox, <a href="http://www.google.com/search?q=cnn">search Google for CNN</a>, view the page source, and search for the keyword <code>prefetch</code>. Mozilla Firefox is the only current browser that supports <code>rel="prefetch"</code>.
 
 <p><a href=http://www.whatwg.org/specs/web-apps/current-work/multipage/links.html#link-type-search>rel="search"</a> &#8220;indicates that the referenced document provides an interface specifically for searching the document and its related resources.&#8221; Specifically, if you want <code>rel="search"</code> to do anything useful, it should point to an <a href=http://www.opensearch.org/>OpenSearch</a> document that describes how a browser could construct a URL to search the current site for a given keyword. OpenSearch (and <code>rel="search"</code> links that point to OpenSearch description documents) has been supported in Microsoft Internet Explorer since version 7 and Mozilla Firefox since version 2.
 
-<p><a href=http://www.whatwg.org/specs/web-apps/current-work/multipage/links.html#link-type-sidebar>rel="sidebar"</a> &#8220;indicates that the referenced document, if retrieved, is intended to be shown in a secondary browsing context (if possible), instead of in the current browsing context.&#8221; What does that mean? In Opera and Mozilla Firefox, it means &#8220;when I click this link, prompt the user to create a bookmark that, when selected from the Bookmarks menu, opens the linked document in a browser sidebar.&#8221; (Opera actually calls it the &#8220;panel&#8221; instead of the &#8220;sidebar.&#8221;) Internet Explorer, Safari, and Chrome ignore <code>rel="sidebar"</code> and just treat it as a regular link. [<a href=http://wearehugh.com/public/2009/04/rel-sidebar.html>rel="sidebar" test case</a>]
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 <p><a href=http://www.whatwg.org/specs/web-apps/current-work/multipage/links.html#link-type-tag>rel="tag"</a> &#8220;indicates that the tag that the referenced document represents applies to the current document.&#8221; Marking up &#8220;tags&#8221; (category keywords) with the <code>rel</code> attribute was <a href=http://www.powazek.com/2005/07/000532.html>invented by Technorati</a> to help them categorize blog posts. Early blogs and tutorials thus referred to them as &#8220;Technorati tags.&#8221; (You read that right: a commercial company convinced the entire world to add metadata that made the company&#8217;s job easier. Nice work if you can get it!) The syntax was later <a href=http://microformats.org/wiki/rel-tag>standardized within the microformats community</a>, where it was simply called <code>rel="tag"</code>. Most blogging systems that allow associating categories, keywords, or tags with individual posts will mark them up with <code>rel="tag"</code> links. Browsers do not do anything special with them; they&#8217;re really designed for search engines to use as a signal of what the page is about.
 
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