Even in desktop Firefox (on linux) I immediately tried to:

- Multi-select by Ctrl+clicking on other items I want selected. (works)

- Shift-click to expands the selection linearly from the focus (works)

- Select one item, then Shift+Up/Down arrows to expand the selection. (Works)

I bet a custom control would be missing most of that behaviour as well.

Mind you, those affordances are completely unknown to most users. I bet if you asked most users (even developers) how to select multiple items in that control, 9/10 would struggle. The input control doesn't make its affordances obvious at all.

So in all, I'm confused what the browser vendors expect people to do here. I'd love to use the built in controls. But as others have said, they're often ugly, they confuse people, their APIs are a mess and they're difficult to style.

Yes it really is. And now imagine someone would instead use a custom multi-select, because their browser has a bad implementation. Actually the best example why custom form elements shouldn‘t be used.

You can still write one as an add-on for your browser and help others or use it as poc for a ticket to the browser vendor.