What Does It All Mean?

1.?In Activating Browser Modes with Doctype,Henri Sivonen summarizes the different modes browser will work in based on thedoctype:

?Both of these techniques still work in HTML5. The HTTP header is the preferred method, and it overrides the <meta> tag if present. And it got a little easier in HTML5:

?Which itself should be encoded in ASCII(http://www.w3.org/TR/html4/charset.html#h-5.2.2)
?But you can also use rel="alternate" in conjunction with other type attributes to indicate the same content in another format, like PDF.

10.?? The correct way to link to translations of documents is to use rel="alternate" in conjunction with the hreflang attribute to specify the language of the linked document.

11. rel="author" is used to link to information about the author of the page. This can be a mailto: address , though it doesn’t have to be. It could simply link to a contact form or “about the author” page.?
and rel="prev" , just like HTML 4, and supports rel="previous" for backward compatibility.
All major browsers support this usage to associate a small icon with the page. Usually it’s displayed in the browser’s location bar next to the URL, or in the browser tab, or both.
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14.?rel="license" indicates that the referenced document provides the copyright license terms under which the current document is provided.
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15.? rel="nofollow" indicates that the link is not endorsed by the original author or publisher of the page, or that the link to the referenced document was included primarily because of a commercial relationship between people affiliated with the two pages. The thinking was that if “nofollow” links did not pass on PageRank, spammers would give up trying to post spam comments on weblogs. That didn’t happen, but rel="nofollow" persists.
16.rel="noreferrer" indicates that no referrer information is to be leaked when following the link.
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17.? rel="search" indicates that the referenced document provides an interface specifically for searching the document and its related resources.” If you want rel="search" to do anything useful, it should point to an OpenSearch
?document that describes how a browser could construct a URL to search the current site for a given keyword.
18.rel="tag" indicates that the tag that the referenced document represents applies to the current document.” Browsers do not do anything special with them; they’re really designed for search engines to use as a signal of what the page is about.
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?You can also reference aminified version:
?The pubdate attribute is a Boolean attribute. It means one of two things. If the <time> element is in an <article> element, it means that this timestamp is the publication date of the article. If the? <time> element is not in an <article> element, it means that this timestamp is the publication date of the entire document.?
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25.?? People with disabilities care about the semantics of site navigation. Once they get past the page title, the next important pieces of information about a page are the major navigation links. If they want to navigate quickly, they’ll tell their screenreader to jump to the navigation bar and start reading. Thus being able to determine navigation links programmatically is important. HTML5 provides a semantic way to mark up navigation sections: the? <nav> element. 
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26.?The HTML5 specification says, “A footer typicallycontains information about its section such as who wrote it, links to related documents,copyright data, and the like.”