Use native progress bar style

Remove border and background properties for the progress bar
from CSS. It takes significant hacking with Mozilla and Webkit
pseudoclasses. Therefore, settle for a simple progress bar for
the time being.
Set the relative sizes of practice panel elements

Ensure that the width of the input box (textarea) as well the
combined width of the three elements in the feedback pane are
each less than the width of the target text element (div).
Additionally, add more space around the feedback pane.

  1. The current input box turns out to be wider than the target
     text element in a high resolution monitor. Set its width in
     percent, so that it is always 95% the width of the target
     text. This ensures that the width of the the input box
     never exceeds that of the target text.
  2. Set the status pane and result pane as inline blocks with
     width set to 30%. The progress pane is already set as
     inline block with width set to 30%. This ensures that the
     total width covered by these three elements is less than
     that of the target text element.
  3. Increase the top margin and bottom margin of the feedback
     pane from 2% to 3%. This provides more space around the
     feedback pane and looks better in a high resolution monitor.