[whatwg] CSS canvas() function

[Tab Atkins Jr.]

(This is being sent to the WHATWG list, rather than the CSSWG list, as
it seems like the sort of thing that should be primarily defined in
HTML, with a CSS spec just referring to the HTML definition, like
:active and similar things.)

Webkit has for some time now supported using the -webkit-canvas()
function in CSS anywhere you could use an image
(<http://webkit.org/blog/176/css-canvas-drawing/>, published in April
2008).  The function takes an ident, which can then be used in the
window.getCSSCanvasContext() function in javascript to grab a context
keyed to that ident.  Drawing on that context is then reflected
everywhere on the page that references that canvas.

In essence, this is defining a scriptable paint server, which seems
useful enough that we'd really like it in the platform.

I'm willing to define a canvas() function in the CSS Image Values
specification, but since this relies explicitly on an HTML feature, it
feels like the full definition should exist in the HTML spec.

Thoughts?

~TJ