Hi WebKit folks,
I have a question to origin restriction and CSS. First the context:
CSS Masking[1] aims to combine the two different 'mask' property
implementations from WebKit and Firefox. To make it short, 'mask' takes an URL
and this can either be a reference to an image, or to an <mask> element:
<svg>
<mask id="mask">
…
</mask>
</svg>
<style>
img {
mask: url(#mask); /* references an SVG Mask element for masking
operation (Firefox), OR */
-webkit-mask: url(test.png); /" takes a reference to an image and
operates the masking with the image (WebKit) */
</style>
<img … />
CSS Masking tries to make both possible in the future.
The problem is, that a <mask> can be in an external SVG file and be referenced
by SVG fragments: url(http://external.com/mask.svg#mask). It would still not be
sure it the fragment points to a graphical element like a <path> (then the SVG
file would be interpreted as image) or if it references a <mask> element, in
which case it will be an external resource. You need to download the file and
parse it to know that.
Does it matter to know if it is an external resource or image before
downloading?
A <mask> element can have events, which run JavaScript: <mask
onload="console.log('CORS? Of course!')">. Running the <mask> element will fire
the event, which is maybe a violation against CORS, since the SVG file can come
from a different origin.
For images, we don't care about the origin, don't run scripts and don't fire
events.
My question is if we have already an CSS property, widely implemented in
WebKit, where CORS matters? When do we decide not to go on with the
interpretation of a resource because of violation of CORS? Before we even
download the resource? After we may already parsed it?
Firefox does care about that before parsing and does not load the entire SVG
file if it is from a different origin. Since Firefox does not support image
references for 'mask', they always assume that the reference is an external
reference and not an image.
Opera does not care about the origin, but scripts are not executed and events
don't fire.
Greetings,
Dirk
[1] http://dvcs.w3.org/hg/FXTF/raw-file/tip/masking/index.html#mask-property
PS: WebKit has support for 'mask' property with referencing of <mask> elements.
Currently, external files are not supported. The mask element must be in the
same document.
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