Definitely not. Why would they let web devs render outside of the browser window? That's a recipe for disaster.
There needs to be some middle ground. I'd trade off just being able to set just the background color and font and keep these native-like features
I think that you can compose arbitrary images that way using an image-like font (using various Unicode chars and assigning to each a small tile of the full image)
Actually I am now curious of how much detail/size/colours/animations you can fit in a single letter.
If that's all your changing, then why can't you make do with the system default?