jarhar:

were "select multiple" and "input type" bad ideas?

Yes, IMHO. At least from the perspective of developer ergonomics. Especially in the case of ”input type”, smuggling 20 different logical elements with different presentations/UI/behavior into one element has created very significant costs that would have been unnecessary otherwise. It's made it more difficult to document, and more difficult to do conformance checking for, more difficult to explain to developers when they run into problems (for example, developers coming to Stack Overflow with questions, and other people trying to help answer them).

I think what I'd look at is how much branching you would need to do and how much shared code there ends up being between the two features. And how difficult it would be to add new features if they were separate elements.

And while it's probably correct that we'd do <input> differently today, the way it was designed has a number of benefits we still benefit from today. It has made it much easier to introduce new input controls, such as type=email. Or soon <input type=checkbox switch>.

y = $0.attachShadow({mode:'open'})
y.append(document.createElement('button'))
x = y.querySelector('button')
x.addEventListener('focusin', console.log)
y.addEventListener('focusin', console.log)
$0.addEventListener('focusin', console.log)
x.dispatchEvent(new FocusEvent('focusin', { relatedTarget: x, bubbles: true, composed: true }))

I only get 2 console.logs when I expected 3