Recently I had to use iframes on a website conforming to XHTML 1.0 Strict. As you might know, the XHTML 1.0 Strict doctype doesn’t allow the use of iframes. The XHTML 1.0 Transitional doctype on the other hand, does allow you to use iframes, but I don’t like to use that doctype. The reason is, as the name implies, that it’s a doctype meant for make the transition from HTML into XHTML – a sort of a temporary solution.

When building new websites I like to use a strict doctype because it doesn’t allow for many of the style and behavioral tags that is much better placed in stylesheets and JavaScript.

What’s needed is a doctype that conforms to XHTML 1.0 Strict which also allows for iframes.

A solution

What I came up with was very simple, but may be considered a hack by some. Basically, I took the doctype declaration (DTD) of XHTML 1.0 Strict and added support for iframes. I found how iframes was supported in the transitional DTD and copied it to the Strict DTD. It allows for some attributes like width and height that the Strict DTD doesn’t, so I removed those and then added support for the allowtransparency attribute.

So to make iframes work on your own invalid XHTML 1.0 Strict page, just replace the doctype at the top of your pages with this new one:


I suggest you download and host the DTD on your own server instead of using mine in case I forget to pay my hosting fee.

Check out the demo of XHTML 1.0 Strict with Iframe

A hack?

Some might say it’s a hack because by using this DTD the page is no longer XHTML 1.0 Strict. That is correct. It is now something new and different, but completely identical to XHTML 1.0 Strict with support for iframes. So it’s XHTML 1.0 Strict with Iframe.

XHTML have build in support for custom DTDs and thus this is completely supported and valid XHTML. If you don’t like using other doctypes than the few main ones created by the W3C then I have to ask why? What does it give you, your users or the quality of the page that this new one doesn’t?

In my book, it comes down to using a doctype that is based on known standards (XHTML 1.0 Strict in this case) so it still make sense to other devs when they read the markup. It’s also important that the DTD is strict (yep, this DTD is still strict), but most of all it’s important that the markup conforms correctly to the DTD so the entire page is valid. Remember, when using custom DTDs your page is still valid XHTML.


I tried using XHTML 1.1 modules to build the DTD, but it never worked out for me. I got to the point where the iframe tag was valid, but not allowed in any other tags including body. I couldn’t seem to find a way to get full support for it. If you know how, please let me know.


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