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We've had about 60 years and multiple huge attempts at it, and IMHO HTML & CSS _are_ the easy to use cross-platform GUI platform. Nothing else stuck--flash, java swing, qt, etc. were all super capable and promising but failed to gain mass usage. HTML & CSS is the way. Nothing in existence has as much capability, developer usage and talent pool, accessibility and internationalization features, etc. A good GUI system i…
I don't think HTML+CSS is the way because of innate goodness of them, but because the web led to so many people being expert at it, and so many tools being built around it, including a cross platform browser or too that are top notch to render it all. At the very least a proper gui library could precompile all of that stuff so you aren't literally parsing HTML and CSS to render things, and HTML And CSS parsers don't…
Don't forget browsers are pretty spectacular runtimes. V8 and its JIT compiler is arguably one of the best runtimes of any language in the world. Sure the very first view of a page is going to do some parsing, etc. but as it runs it gets faster and faster with core functions and components compiled on the fly into platform machine code. The sandbox and security and encryption support in browsers is top notch and supremely battle-tested and hardened. With WASM now pretty mainstream we're starting to see entirely new frontend UIs coded in languages like C++, Go, Rust, etc. that are incredibly fast too. If you squint hard enough the browser is really no different than the JVM or .NET CLR these days--it just has 20 more years or so and an order of magnitude more developers working on improving it.