Of all the most popular editors, I think none of them had a way of offering diagnostics for vanilla HTML. The only exception that I know of is Webstorm.
Vim, Neovim, Helix, Zed, VSCode all shared the same basic implementation that has no diagnostics support.
I wrote IDEs not editors, and I am quite sure that the years doing ASP.NET and Java EE/Spring development I had enough HTML diagnostics to fix since 2001.
Vim, Neovim, Helix, Zed, VSCode all shared the same basic implementation that has no diagnostics support.
Helix will have SuperHTML enabled by default starting from the next release: https://github.com/helix-editor/helix/pull/11609