>When someone says HTML & CSS is terrible I ask them to design something better that lets you represent user interfaces that can adapt across multiple screen sizes, and allow complex layouts that Flexbox and CSS Grid makes possible

Short answer: layout and styling in one DSL with one syntax/grammar and not you know, two. But the problem isn't "how do I make something that's better" but "how do I make something that's better and supported on my target platforms."

If browser engine support isn't a design concern there are plenty of alternatives, many of which are arguably superior to HTML + CSS.