This might not be relevant to the topic(I don't use Comcast), but Im curious how web elements are forced to be overlay consistently across all websites, doesn't the existing css properties on the page affect the behaviour of the injected scripts?
It's done via the 'z-index' CSS property [0]. Essentially, the higher the z-index, the higher the priority of the element; a higher z-index will appear in front of a lower z-index.