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Re: [fxtf-drafts] [filter-effects-1] What is the visual effect of filter() on the document element?

From: Amelia Bellamy-Royds via GitHub <sysbot+gh@w3.org>
Date: Mon, 30 Apr 2018 16:57:30 +0000
To: public-fxtf-archive@w3.org
Message-ID: <issue_comment.created-385460652-1525107449-sysbot+gh@w3.org>

The default background is traditionally customizable (e.g., Firefox lets you set this as a user style).  

[CSS Backgrounds](https://drafts.csswg.org/css-backgrounds/#special-backgrounds) says:

> The document canvas is the infinite surface over which the document is rendered. ... If the canvas background is not opaque, what shows through is UA-dependent.

However, [WHATWG HTML says](https://html.spec.whatwg.org/multipage/rendering.html#phrasing-content-3):

> The initial value for the 'color' property is expected to be black. The initial value for the 'background-color' property is expected to be 'transparent'. The canvas's background is expected to be white.

Elsewhere, [CSS Backgrounds says](https://drafts.csswg.org/css-backgrounds/#root-background) that a background on the root element or HTML body element is propagated to the canvas.  So it is theoretically possible to insist that "the background that shows through from a transparent canvas" is white, while also allowing customization in the form of a `:root {background-color: <user-setting>}` style rule. The difference would be revealed if an author style sets a (semi-)transparent root background: does it override the user setting, or layer on top of it? 

Firefox implements it using the layered approach: the user-defined color is applied underneath any background set on the root element.

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Received on Monday, 30 April 2018 16:57:36 UTC

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