Specifying css origin as USER in declarative/registered content scripts

[tophf]

There should be a way to specify origin: 'USER' of CSS content scripts that are 1) declared in manifest.json, 2) registered via chrome.scripting.registerContentScripts.

It would align the API with the existing world parameter for js content scripts.

Why is it important? Because the USER origin is the only way to override !important in inline style attributes of a web page. Currently we can only use chrome.scripting.insertCSS for that, but it's not declarative so it requires observing navigation, which is inefficient as it wakes up the service worker frequently, it may also easily cause a flash of unstyled content if the page loaded before the SW started and the browser processed the API call.

[xeenon]

Extensions styles are already user level in Safari. I am supportive of having a way to change between user and author.

[Rob--W]

Firefox bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1679997

While there is consensus on the capability, I believe that we haven't talked about the exact shape of the API yet.

The tabs.insertCSS API, which is the origin of the feature, takes a cssOrigin property with the values "author" or "user": https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/insertCSS#cssorigin

The scripting.insertCSS API has renamed the name to just origin with the values "AUTHOR" or "USER" (upper case): https://developer.chrome.com/docs/extensions/reference/scripting/#property-CSSInjection-origin

Because this feature makes sense in manifest.json and scripting.registerContentScripts, just "origin" would be confusing, so I propose to stick with the cssOrigin name.

Since the scripting namespace is already using the upper case variants of the value, I would continue that practice in the improved API.

[xeenon]

I plan to add support for css_origin with author and user values from the manifest.json in Safari / WebKit. I went with snake case for the key and lowercase for the values for consistency. And cssOrigin to scripting.registerContentScripts() with author and user values (case-insensitive since scripting already has AUTHOR and USER values for the origin key of insertCSS().) See #563 for discussion on case.

[xeenon]

WebKit issue tracking this: https://bugs.webkit.org/show_bug.cgi?id=273122