Monday, January 11, 2010

Chunking away (at rule languages)

I want to demo my formalized+parallel CSS work in Tahoe next Weds (Thurs?) so I've been hacking away the last few days to put in some missing pieces. I wrote the basic cascade algorithm and moved on to translating from CSS to my (parallel) kernel language... and realized I had an ambiguity. Well, there are many -- e.g., what happens with '!important' declarations in user agent style sheets? -- but one that shouldn't have existed given a principled approach to language design.

Consider the following page:


div {
background-color: gray;
background: pink;
}
p {
background: pink;
background-color: gray;
}
...

<div> hello
<p> world


What are the background colors of the dividing box and of the paragraph? Pink and then gray. Instead of using the most specific declaration in a set of declarations, the last applicable one is used. The *only* time I would have thought that would happen is if there were conflicting most-specific definitions. But no, specificity is only a notion applied to selectors, not extended to describe what they're assigning. Now I need to back-pedal and propagate lexical positions of declarations (in theory... not a priority). Not as bad as dealing with hoisting in JavaScript, but another one of those gotchas.

I'm finding a lot of these little things. They're not earth shattering, and seem mostly due to inertia. However, especially when I'm dealing with actual layout rules, stuff is often flat out undefined -- though these points of ambiguity might also be an opportunity (e.g., pick a normative definition that facilitates parallelism ;-)).
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