What determines if an extension UI can be modified by userContent.css?

Add-ons Development
#1

In Firefox 57, the UI of some extensions can be modified by userContent.css, whereas the UI of some extensions cannot.

What determines this?

As examples, the UI (including the dropdown) of uBlock Origin can (gratefully) be modified, but the UI of Page Saver WE cannot.

For Page Saver WE, the following userContent.css code has no effect (replace the UUID as needed):

@-moz-document url(moz-extension://[UUID]/popup/popup.html) {
   #sendFeedback { display: none !important; }
}
#2

I have absolutely no idea why, but for some reason, Firefox doesn’t apply #sendFeedback. All other selectors I tried, including other id selectors, work as expected. This should to what you want:

@-moz-document
	regexp("moz-extension://[\w-]+/popup/popup.html")
{
	button[i18n-msg="sendFeedback"]
	{ display: none; }
}