Generally aligning stuff isn’t nice. But if you do, it’s tabs up to whatever level of indentation you’re at then spaces the rest of the way. So you wouldn’t have to assume a tab size. And the tabs and spaces have different semantic meaning (indent vs alignment) so mixing them makes sense.

(Dashes for tabs, * for spaces)

<form>
—<input type=“text”
—*******class=“whatever” />
</form>

Although really just adding a level of indent is better than aligning.

<form>
—<input type=“text”
——class=“whatever” />
</form>