Attribute change steps should run after setting the value, not before #1190
Description
https://dom.spec.whatwg.org/#concept-element-attributes-change first runs the change steps, then sets the value.
That means that if the steps trigger something that then changes the relevant attribute, per spec we should keep the "old" value, which seems wrong.
Example:
<!doctype html>
<div popover="manual">wat</div>
<button>Hide</button>
<script>
let div = document.querySelector("div");
div.addEventListener("beforetoggle", function(e) {
if (e.newState == "closed") {
this.setAttribute("popover", "manual");
}
})
document.querySelector("button").addEventListener("click", function(e) {
div.removeAttribute("popover");
});
div.showPopover();
</script>Per spec, after clicking the value the popover attribute should not be there, which is wrong and doesn't match implementations.
My guess is that modulo that case this is generally not very observable, because other attribute change steps are usually async.
It feels very weird tho that removeAttribute runs script synchronously for popover btw...
cc @mbrodesser-igalia, @mfreed7, @domenic, @annevk, @smaug----