[Web] Please support a custom stylesheet for platform views

[sma]

Flutter Web encapsulates all HTML created via HtmlElementView in a shadow DOM of a custom flt-platform-view element. The shadow DOM automatically get a style reset attached before the HTML element is added. This makes external customization of the HTML very difficult if not impossible. This is by design, I guess, but I'd like to globally customize the styles via an external .css file and not use inline styles for my HTML in each and every HtmlElementView.

Therefore, please support overwriting the all: initial style reset with a different style definition. An easy way would be to pass with ShadowRoot via getCreatedView to the callback, so that I can remove the reset and add my own style elements.

[ditman]

We're doing some work to make the Shadow DOM optional; this is a good new feature to add to that work. Let's link this two issues together:

#50452

(Also see this comment on a way you might be able to append a style tag within the ShadowDOM)

[ditman]

Platform views now live OUTSIDE the shadow DOM, so they can be styled with standard CSS in the index.html of one's app. This shouldn't be an issue any longer.

Closing, but feel free to reopen if there's still trouble with this!