Question

I like the data-* tags concept.

Should i use it in non HTML 5 documents?
For example:

<li data-id="1001">Item A</li>

For indicating the inner application is (rather than the DOM id)

Answer 1

The only difference is that your document won’t validate as HTML 4.01 (or whatever non-HTML5 version you’d be using). Nothing would actually break; browsers won’t care.

Here’s a relevant quote by Paul Irish (source):

The validator giving you errors doesn’t mean browsers won’t render
your page as well as they can. In fact, what the validator thinks of
your markup is very very different from what a browser thinks of it.

In fact, it’s more like a browser is your friend who is super
flexible, even when you were late to hanging out with him and
everything you say to him sounds like a jumbled mess… He’ll totally
understand and be as accommodating with your imperfections as
possible.

Mrs Validator, on the other hand, will mark up your papers
with red pen again and again. Yes your thesis is perfect and it’s
supported with paragraphs of justification masterfully. Such prose,
such rhetoric! But no you forgot a comma and your “who” should be a
“whom” and that semicolon absolutely should be an em-dash. Do it
again! Meanie. 🙁

Validation can be a useful tool to make sure you
haven’t made any egregious errors. But don’t treat 100% valid markup
as a goal. There are plenty of extremely well justified decisions you
can make in your markup that will not validate, like the
X-UA-Compatible meta tag. For more thoughts on validation, I
certainly recommend Nicholas Zakas’s post:
http://www.nczonline.net/blog/2010/08/17/the-value-of-html-validation/

PS. as an example of how browsers are super accommodating.. You can
use all sorts of HTML5 features even with an XHTML 1.0 Strict
doctype. Browsers don’t really give a shit. They make it work.