Found out some information myself: (1) multi-threading for algorithms in VTK causes the calling thread to wait for their results with pthread_join(). This is discouraged in emscripten leads to warnings unless explicitly allowed, so I rather chose to use PROXY_TO_PTHREAD. (2) A canvas element that is changed to offscreen indeed loses with and height attributes (they are available at the offscreen canvas then) but not their corrresponding css elements.