Using HTML5 elements with XHTML doctype

Question

I'm part of a team that's updating a website for a bank.

The web pages use an XHTML doctype:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 
         "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

However, I'm finding lots of HTML5 elements in the source and they want more.

Updating to the HTML5 doctype is not an option.

According to the bank, the site works perfectly well as-is. So apparently the browsers don't seem to care (although the pages are validating with lots of errors).

Is it safe to just skip the doctype issue?

Is there a standard method for handling HTML5 elements when using pre-HTML5 doctypes?

I'm looking for any potential security or performance issues.

The doctype s a suggestion of what rules to follow when rendering a page. If it finds that the page doesn't conform to the strict rule is drops to the next level down. If it finds valid html5 elements, and the browser can render them, then the page will render them but maybe in quirks mode. — user2417483
– user2417483, Commented Oct 27, 2015 at 17:51
@jeff The doctype is a definition, not a suggestion. That's why it's called the "Document Type Definition". — Rob
– Rob, Commented Oct 27, 2015 at 18:00
Is the XHTML actually served as "application/xhtml+xml"? Or is it served as "text/html"? — Rob
– Rob, Commented Oct 27, 2015 at 18:02
@Rob, content="text/html; ..., as expected. (I don't think I've ever seen the elusive application/xhtml+xml in the wild.) — Michael Benjamin
– Michael Benjamin, Commented Oct 27, 2015 at 18:04

Rob · Accepted Answer · 2015-10-27 18:13:13Z

1

As you stated in the comments, you are serving your "XHTML" as HTML, and not true XHTML in the first place. I don't recall if this comes under the definition of "tag soup".

The new standard doctype is backwards compatible for older browsers and works as far back as IE6; maybe even IE4. Since you are already serving HTML, and not XHTML, you shouldn't have issues with changing to the new one but the only way to be sure is to test it.

However, HTML is HTML no matter which doctype you use, and that's for validation purposes only.

The reason browsers will work with your HTML5 markup is because they are required to "do the best they can" with invalid markup. Since HTML5 is not invalid, and they know how to handle it, they do their best.

There are no security or performance issues.

It might help to read this article: Activating Browser Modes with Doctype

edited Oct 27, 2015 at 18:13

answered Oct 27, 2015 at 18:08

Rob

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