Yes. Any HTML tag that your browser supports will work in the box. So you can carry tags from chapters to chapters.

Yes, so long as what they generate ends up as part of an XML-conformant file (ie either valid or just well-formed).
Server-side tag-replacers like shtml, PHP, JSP, ASP, Zope, etc store almost-valid files using comments, Processing Instructions, or non-XML markup, which gets replaced at the point of service by text or XML markup (it is unclear why some of these systems use non-HTML/XML markup).

In HTML, a tag tells the browser what to do. When you write an HTML page, you enter tags for many reasons -- to change the appearance of text, to show a graphic, or to make a link to another page.