Arguably I prefer to have the option to use software that underperforms than to not be able use it all all, specially if performance is not that critical.
1. Detect GPU acceleration
2. With acceleration, enable fancy bells-and-whistles, with animations and effects
3. Without acceleration, fall back to good-old-fashioned mode, with flat HTML
A lot of this depends on what YOUR software does. Is it speeding up a spreadsheet (which will probably work fine in CPU)? Is it adding fancier shaders to a video game?
Explicit beats implicit. The library should support CPU and GPU, without a doubt, but the user of the library should make the decision of what to do if GPU isn't found.
If you're writing a spreadsheet, use a fallback to CPU. If you're writing a glitzy animation, turn it off.
Shoot, Blender still supports users without accelerated graphics. (Or did five years ago, things may have changed.)
Moreover, it's not up to you to decide what's acceptable or not to a 3rd party, specially if your claim is being made to justify choosing between being able to use something that performs below par or not be able to use it at all.