There is nothing wrong with the sentence. It's perfectly grammatical.
Let's see one example
- A doctor is what he would have been had he had enough money to put himself through medical college. (If he had enough money to pay for the medical college fees, he would have been a doctor. Meaning he is not a doctor today.)
In your sentence (I'm not a technical person so i don't know what Angular is) it means that
If HTML had been designed for applications it would have been Angular. (It means that current HTML is not designed for applications)
We should put a comma to make that sentence more understandable like
Angular is what HTML would have been, had it been designed for applications.