Not too long after Windows 8 launched with the AC-3 and E-AC-3 codecs, Internet Explorer has had the capability to decode these audio formats. It recognizes the FourCC codes in the ISO Base Media File Format container as well as the MIME tags.
Part of the problem, however, is the perceived shift in both audio loudness and the perceived location of speech. All AC-3 and E-AC-3 content, when properly measured, should play dialog back at -31dB relative to full digital scale. Unfortunately, this makes the codec inherently quieter unless the decoder is set to something called RF mode, which boosts the loudness to -20dB and compresses the audio more heavily. Such control for loudness is not typically found in HTML5-based apps, though the W3C has a committee working on this issue. The loudness can be a particular problem on the Windows 8 tablet devices out there, as many programs in AAC format come pre-normalized to somewhere around -23dB to -24dB relative to full scale. Unless all content is pre-normalized to the same consistent playback level - which AAC ads will definitely not be, and probably not AAC stereo content - there will be an inconsistency of experience.
All of this also presupposes that you have either a proper surround virtualizer or a discrete 5.1 speaker system such as is found in a properly set up home theater. Considering that less than a third of homes have any kind of surround sound in them, and given the loudness issues, I'm not certain what the benefits will be here. But it gets even worse, as dialog in multichannel AC-3 and E-AC-3 is steered to the center channel in most programs, whereas in stereo content it is mixed into left and right without regard to position. This can result in disturbance to the listener. Furthermore, any channel configuration changes to an audio-video receiver will typically cause muting when switching modes between stereo output and multichannel output, potentially interrupting the experience for the listener.
Part of this is the add-on nature of AC-3 and E-AC-3 to Windows and an inherent failure to integrate stereo AAC and HE AAC playback behavior with that of stereo and multichannel AC-3 and E-AC-3. Until then, this will be more of a curiosity than anything substantially improving the consumer's experience, and developers should take note if they believe that HTML5/CSS/Javascript development of their apps can really unify their experience across devices yet.