Let generated HTML files include a DOCTYPE declaration

Hamlet makes this easy, and it seems good citizens do this kind of thing.
Allow defining a custom onload action per generated HTML page

This will be useful to define a piece of JavaScript to be executed
when loading a page, so that I can automatically focus certain
elements or select their text.
Automatically focus 'URL' input field when loading web page

This is convenient for keyboard-heavy use cases: you can just load
the web page (i.e. via a bookmark) and then start typing (or press
Ctrl+V to paste) right away.

Tested the JavaScript snippet in Chrome (the only browser at hand),
seems to work fine.
Automatically selected text of shortened URL

This allows pressing Ctrl+C right away to copy & paste the text, which is what
people probably usally do anyway since they most certainly cannot just type the
shortened URL due to the funky Unicode characters.

The JavaScript function which does the (supposedly somewhat portable)
runtime text selection was taken from a StackOverflow article.