For large files it's better to have a viewport because
then you can view both the form as well as the part of
the output you are interested in at the same time.

* "Try out Pygments!" is more descriptive than "Try it out!"
* Moved the description of the implementation to the end.
* Added a <noscript> tag suggesting `pip install` and pygmentize.
* Added a <label> for the textarea.
* Disable spell checking for the textarea.
* Reduced the scope of the reset button to the file input.
* Added autofocus for the language select.