What problem does this feature solve?

solves a parse failure in https://github.com/vitejs/vite/issues/5966 according to mentioned issue, duplicate class attributes are allowed in html

related issue: https://github.com/vuejs/vue-next/issues/4883

Reproduction

What does the proposed API look like?

a new option to to allow duplicate attributes

https://html.spec.whatwg.org/multipage/syntax.html#attributes-2

I wonder in what standard duplicate attributes are allowed.