Drawing a line is always hard (when did in our evolution we become sentient?), but HTML / CSS is probably the furthest you can be from that line in modern UI tooling.
As long as the UI or it's underlying engine was built with GPU in mind and not as an after-thought to accelerate it, it can be safe to say it probably fit the "GPU-Backed UI" definition.
As long as the UI or it's underlying engine was built with GPU in mind and not as an after-thought to accelerate it, it can be safe to say it probably fit the "GPU-Backed UI" definition.
If you can easily attach a Shader (https://github.com/emilk/egui/blob/master/examples/custom_3d...), it is also a dead giveaway that it is a "GPU-Backed UI".
WebGL is different from HTML / CSS. You could have a "GPU-Backed UI" running in WebGL.