Finally get rid of the archaic <table> markup for header and footer lines

Finally get rid of the archaic <table> markup for header and footer lines
and use flexbox CSS instead.  Improve accessibility by adding role
and aria-label attributes to these header and footer lines.
Using ideas from both Anna Vyalkova <cyber at sysrq dot in> and myself.

As a welcome side effect, this also resolves the long-standing issue
that the rendering was always 65em wide, requiring horizontal scrolling
when the window was narrower.  Now, rendering nicely adapts to browser
windows of arbitrary narrowness.