问题:

Is it correct practice or valid syntax to use other tags inside a <title>?

An example for multi-language title

<html lang=en>
<title>Some title in English and a <i lang=fr>word in French</i></title>

回答1:

See http://www.w3.org/TR/html401/struct/global.html#h-7.4.2:

Titles may contain character entities (for accented characters, special characters, etc.), but may not contain other markup (including comments).

(my emphasis)

回答2:

No, it may not

http://www.w3.org/Provider/Style/TITLE.html

回答3:

You can try to use whatever you want, but it will all be used as title string, without any additional parsing/processing from the browser (if that's what you expect). RFC says you have to resist from placing markup in title, though.

回答4:

TLDR: The <title> tag (1) must contain text (it must not be empty), (2) must only contain text (i.e. no other elements), and (3) must contain text that is not just white-space.

In HTML 5, the Content Model of the title element is: