Chromium browsers compiled without proprietary codecs require webm. It's a small subset of people (I am one), but I just don't play and/or leave sites with mp4 only.
It is kinda-sorta free and open source, though (at least the decoding part). Cisco pays for their codec, and as long as you use their implementation you are covered by their license.
Specifically, to be covered you have to download a binary release from https://github.com/cisco/openh264/releases during the installation to be covered by Cisco's MPEG LA patent license, but they provide builds for most major platforms.
