Browser based conferencing and accessibility

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I am currently investigating online conferencing systems. We have been using these systems at my institution for several years but the one we use requires a fairly hefty download and things do not always work optimally and for this we pay a substantial amount of money.

So now I am looking for something else and one of the primary requirements is that it should be browser-based so that participants at least (if not the presenters or meeting chairs) find it as easy to attend an online meeting as it is to surf the internet.

But here's the rub. Most, if not all,