Use FFMPEG as an easy way to export your After Effects video for use in a web browser. Animated GIF was 5MB, MP4 with H.264 was 1.1MB, and WebM was only 230KB!
Use Media Encoder to export to PNGs with Alpha Channel. Then use the following command to to combine PNGs to webm file. Note that %03d should reflect the number of digits in your file name on export. If you video has 4 digits, this should be %04d.
Command to use: ffmpeg -i "yourfilename%03d.png" output.webm
Cheers!