Textarea Span

The textarea needs its own span because

• to be focusable,
  • it has to be "visible",
    • which means it must have nonzero width and height,
• but we don't want it to affect layout of the math,
  • so we want to take it out of normal flow by giving it position:absolute,
• but we want it in corner of the math editable because many browsers will scroll to the x,y position of the focusee on focus,
  • and we can't be sure the root math element is the containing block,
  • so we have a span that's position:relative and thus will be the containing block
    • and that's in normal flow and thus is in the corner of the math editable