For instance, Firefox’s #browser-bottombox has it by default.
There’s practically zero documentation on it. I can tell it somehow creates/delimits an area of the browser to be rendered “independently” from the rest, but I have no idea of what it does exactly, or the implications it brings for other adjacent elements in the DOM.
My problem: I’ve got to set layer=“true” in the tabbrowser#content for a feature in one of my add-ons to work properly with other similar features of my other add-ons. And as far as my add-ons are concerned, everything seems to work fine, but because I don’t know the exact nature of the implementation of this attribute, I also can’t tell beforehand what kind of things this can possibly break (in Firefox or in other add-ons), or if anything will break at all.
Does anyone have a clue?