Notes and Frequently Asked Questions
Firefox - Firefox supports AAC audio playback in HTML5 only on hardware platforms or operating systems that have AAC support built-in. Thus, Firefox on Windows, OS X, or Android will play AAC audio since those operating systems include an AAC decoder. For Firefox on Linux, Gstreamer must be installed from the "non-free" repository of your distribution. For more information see footnote  of https://developer.mozilla.org/en-US/docs/Web/HTML/Supported_media_formats.
What is it supposed to sound like? - In cases where this is not self-evident, or explained by comments on the page, the uncompresssed signal may be downloaded for comparison.
Nothing happens when I click the Play triangle or the player for an item seems to dissappear sometimes - Two things could be happening: One issue is some of the files are moderately large. Many wav files on this site are 5 to 25 MB in size. On a slow or mobile connection, it may take some time for the files to finish downloading and be playable. The other issue is the file may have downloaded into your web browser, but the browser is not able to play it. In some cases, the HTML5 player element will not be shown in this case.
Do these tests guarantee conformance? - No, they are intended as informal tests to check implementations. MPEG and Fraunhofer have formal conformance streams available for complete testing.
If a feature/browser is said not to work here, is that true? It might have been at a point in time, but this website is not routinely updated. You may want to use the streams here to recheck this.