Generate HTML5 instead of HTML 4.01

[jwakely]

• Use HTML5 DOCTYPE.
• Use CSS to style tables instead of obsolete HTML attributes.
• Use id attributes instead of name. Sanitize the values of all such attributes (and corresponding URLs using them) so that they don't contain spaces.
• Do not create duplicate id attributes in index pages that aren't sorted by section.
• Postprocess issues to replace obsolete <tt> elements with <code>.

There are validation errors for some of the issues themselves, due to the use of <p/> as a paragraph break, which is not correct HTML5. Paragraphs should start with <p> and end with </p>, not just be started implictly and then separated by <p/>.

Wrong:

This is paragraph one.
<p/>
This is paragraph two.

Also wrong:

<p>
This is paragraph one.
<p/>
This is paragraph two.
</p>

Right:

<p>
This is paragraph one.
</p>
<p>
This is paragraph two.
</p>

[jwakely]

There are validation errors for some of the issues themselves, due to the use of <p/> as a paragraph break, which is not correct HTML5. Paragraphs should start with <p> and end with </p>, not just be started implictly and then separated by <p/>.

To be clear, these are pretty harmless. Browsers do the right thing. It's only a problem if you're using https://validator.w3.org and trying to get rid of all warnings and errors.