Even though html5 exporter comes with the options "resize display to fill window size" and "Fit inside (black bars)". "Fit inside (black bars)" makes no difference whether on or off.
It's only possible to resize the to entire size of the browser window, thus not keeping aspect ratio.
For resizing to have any use, I would like to suggest that a way be added to keep aspect ratio on resizing. It could be made simply by making the "Fit inside (black bars)" work.
Putting the html5 canvas in an iframe and scaling that, is not an option for me.