Changing markup of elements from markdown

My blog has a fixed width, and uses internal side-to-side scrolling for fenced code blocks (overflow-x: auto.) Unfortunately, this creates an accessibility violation! You can read about it here:

The fix here is to add tabindex="0" to the pre elements. I can do this myself with a regular expression, of course, but is there a way to override the output during generation?

Not possible no and not planned.