A common request for partner engineering is about supported codecs. Partners without device often tested upfront using the Simulator, finding that MP3 or MP4 don't work and therefor having doubts about feasibility on their app on Firefox OS.
The simulator should point out in some way that video and audio are limited to whats supported on Desktop Firefox and that Firefox OS and Android have additional codec capabilities (MP4, WebM, MP3 and AAC) depending on device hardware.
As one possible idea, this might be during first launch, a note that the user can hide.
The Simulator has a number of such limitations, each of which applies to only a subset of developers, so I'm wary of notifying all users about them on firstrun, as it could feel like information overload or just an annoyance that developers need to click through to get to what they want.
Nevertheless, you're absolutely right that we should let the relevant developers know about these limitations, so they aren't confused by them! At the very least, they should be documented on a page that lists the various capabilities of Firefox OS (including all its WebAPIs in addition to these media formats) and the status of the Simulator's support for them.
Over and above that, I wonder if we could extend @rpl's manifest validator to check for the use of these codecs and include a warning in the validation results when apps use them. That way we don't warn every developer, and we don't warn them before they've even started using the Simulator; but rather we warn just the subset of developers who are using the codecs, and we warn them only once they start using them. So the warning is contextual and targeted.
Having a warning in context would be great, I thought about that. Sadly the manifest doesn't give hints on codec dependencies use, except when asking for audio-context permissions.
I guess we could show it as soon as the user creates an Audio tag instance.
But as a general overview, a Help page might help. I'll go ahead and create a wiki page to start documenting: https://github.com/mozilla/r2d2b2g/wiki/Limitations
I like the wiki page! Note also that don't have to restrict the validator to the manifest. We can extend it to look at other addon files, including its HTML and JS files. So we could search those for audio tags and warn the developer if we find one that the Simulator can't handle.
Just a note: you don't specify what codecs you plan on using in your manifest. Even analyzing the source is problematic because the container format does not tell you what codec is used. example: The ogg contaner (.ogg files) can use all of these codecs.