Most the examples don't list basic file globing for input... so here it is:
$ ffmpeg -pattern_type glob -i '*.png' -c:v libvpx-vp9 -pix_fmt yuva420p output.webm
When new enough ffmpeg can do vp8 with transparency that is backwards compatible with decoders that DON'T support alpha channel:
$ ffmpeg -pattern_type glob -i '*.png' -c:v libvpx -pix_fmt yuva420p -metadata:s:v:0 alpha_mode="1" output.webm
Most examples show something like this:
$ ffmpeg -framerate 25 -f image2 -i frames/%03d.png -c:v libvpx -pix_fmt yuva420p output.webm
But then you have to get write the correct sprintf-like %03d format correct.