If you stick to web standards, like W3C HTML, your content becomes ADA 508 conformant if you follow some simple rules. Beyond that, the benefit of W3C HTML is that your content becomes searchable, deliverable on multiple platforms (e.g. PDAs), deliverable without a lot of plug-ins, and faster to download.
Most plug-in based content is inaccessible to blind
readers. 508 should be about making the content accessible to everyone
at the same time, not about certain pages accessible to blind people
with 3 other pages accessible to hearing impaired only. Also, one of the
best features of Sesame Street (and other shows including my favorite,
"Between the Lions") is that they can offer repetition of presentation
of material without requiring a live instructor. Web based training can
also offer this (if the reader is interested in the topic).