If you dig through my comment history, you might be able to find a comment I wrote on my first exposure to this term maybe ~2 years ago. I was utterly befuddled because I thought it was a graphics rendering algorithm (setting pixels in a framebuffer corresponding to the interior of a polygon specified as an edge list), and was horribly confused about why an article discussing some web technology would suddenly transition into a geometric problem utterly disconnected from the original topic.
I'd be interested to know where/when this "polyfillable" term came about. It's not readily Google-able yet.