Hixie's Natural Log

2006-01-20 07:03 UTC Tag Soup: Crazy parsing adventures

My current main project at work is writing the HTML5 Parser Specification, which is a specification of how browsers should parse HTML, including all the fancy error handling logic that up to now has been left officially undefined.

Of course, although it has been left undefined as far as the specifications go, any browser that wants to even attempt to render any useful portion of the Web quickly finds itself reverse-engineering all the other browsers in an attempt to render the existing content, because, to a rough approximation, all the content on the Web is errorneous, invalid, or non-conformant.

This reverse-engineering has been imperfect, though, since it's not exactly easy to determine how another browser works when you have an infinite range of possible inputs.

I've looked at how browsers handle tag soup before, but now that I'm writing a spec for this, I've run into a host of other issues, and I finally just wrote a tool to see how browsers parsed HTML. It shows your input, the DOM view, the rendered HTML view, and the results of the DOM Level 0 innerHTML attribute, as well as the compatMode state and what the browser thinks is the document title. It's a useful tool, because it lets you rapidly check theories, instead of having to go through the normal edit-save-reload-examine cycle.

I've found some interesting things I'd like to show you.

To explain the notation I'm going to be using, let's start with something simple:

<!DOCTYPE HTML><title>Hello World</title><p title="example">Some text.</p><!-- A comment. -->

In a compliant browser, the DOM tree would be as follows:

Hopefully this should give you a good idea of the various kinds of nodes you'll see in a DOM tree — DOCTYPEs, elements, text nodes, attributes, and comments.

Once you understand the general scheme, consider this: the markup above doesn't look like that in IE! Nor Safari! Nor Opera! In fact, the only browser that gets it right is Firefox!

IE does this:

First, it mangles the DOCTYPE node, instead treating <!DOCTYPE HTML> as <!--CTYPE HT--> (that is, as a comment, cutting the text as if it was a real comment). Then, it adds an empty implied TITLE element, for no apparent reason (the specs require implied HTML, HEAD, and BODY elements, but not an implied TITLE element).

Opera does this:

No implied HEAD element at all! It takes a perfectly compliant HTML document and makes a non-compliant DOM out of it. Oops. This can cause all kinds of problems with CSS and scripting, too.

Not to be outdone, Safari gives us this minimal approach:

That's right: no DOCTYPE, no comment. Still, on a positive note, at least what it does have is correct!

Firefox itself is far from perfect, of course. Let's explore some of Firefox's more amusing... features. First, a lone <TEXTAREA> element:

<!DOCTYPE html><textarea>

You would expect to get simply a blank <TEXTAREA> element, but in Firefox, this is parsed as:

Um. Where did that text node come from?! It turns out that in Firefox, document.close() will always just append </HTML> to whatever was passed to document.write(), whether that makes sense or not. Oops!

How about duplicate attributes? Does the first attribute win, or does the second attribute win?

<!DOCTYPE html><html id="a" id="b"><body id="a" id="b">

Will the first win, or the second? I'll give you a hint. In every other browser, the first one wins. So what do you think? The second, maybe?

If only. It turns out that the first attribute wins, except on the <HTML> tag, where the second attribute wins!

How about bogus elements?

<!DOCTYPE html><html><body><x foo bar>


We'll gloss over the mysterious _moz-userdefined attribute, because it isn't harmful (it's marked with a vendor-specific prefix, and is just used internally so that the browser and editor Mozilla components can recognise real HTML elements from bogus elements). Other than that, it makes sense. But what happens if we remove the (optional) <BODY> tag?

<!DOCTYPE html><html><x foo bar>

It does the same thing, right?

Yeah, nothing surpr— wait! Where did our foo attribute go?!

Finally let's look at what happens when you play with the <OPTGROUP> tag:

<!DOCTYPE html><div><optgroup>

The DOM is:

Curious, you might think, the <OPTGROUP> tag didn't get an element, it was just thrown away. Well, that makes sense; the element is useless outside of a <SELECT> element anyway. Let's remove the completely unrelated <DIV> tag...:

<!DOCTYPE html><optgroup>

We should just get the same DOM with the <DIV> element removed, right? Er, nope:

Don't ask me where those elements came from.

Internet Explorer is not immune from these crazy parsing antics either, of course. Take this example:

<!DOCTYPE html><frameset><frameset rows="1">

It gets this DOM:

The second <FRAMESET> tag gets dropped, but frankly that's not that surprising: around <FRAMESET> elements a lot of things end up dropped. No, what is surprising, is that if you change it like this (just adding a comma on the end of the attribute's value):

<!DOCTYPE html><frameset><frameset rows="1,">

The DOM suddenly changes!:

There's our second <FRAMESET>! Yup, it seems that in IE, the attributes can affect what elements appear in the DOM. Go figure.

Talking about <FRAMESET>s, Safari has an interesting take on this example:

<!DOCTYPE html><body><frameset>

It does this:

I don't recall putting a style attribute anywhere! Where did it come from?

Luckily, when the parsers don't agree, it's usually a sign that no pages depend on their behaviour; so, ironically, every time I find the browsers disagreeing on how to parse some HTML, it gives me more leeway to make the spec sane. Who knows, in a few years, we might have all the browsers parsing HTML the same way! That's certainly my aim. It would make Web development a heck of a lot easier and more predictable.

Note: In all these examples I'm using the HTML5 DOCTYPE (<!DOCTYPE HTML>), which triggers standards mode. The HTML5 spec says that if you use another DOCTYPE, UAs can switch to quirks mode, in which case all bets are off. I'm not even going to try to specify quirks mode parsing. Hopefully, by making the DOCTYPE short and memorable, it will encourage authors to use it more.

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