Accessibility is important and the good thing is Mesop is built on Angular Material components (https://material.angular.io/components/) which has invested in supporting a11y. Mesop components are likely missing aria-* attributes, but it's pretty easy for us to add them (essentially plumbing them from Python into the Angular components), so please file an issue on our GitHub repo to let us know what we're missing, thanks!
At a first glance at the demo gallery, nothing there is labeled as a button except for some in the embedded frame. The buttons there does not have an accessible label though. Nothing is list or grid, on the page and when any of the demo buttons are pressed, the focus is not moved to the new content. So, aria-role and aria-label as a start, I think.
