* id="grid" should be id="class" (since "grid" is not unique in the document).
* Remove space after 'id=' (multiple places).
* Group 3 texture-atlas elements in new <div class="texture-atlas-container">.

* Make terminal take 2/3 of the width, and the options etc get 1/3
* Remove options height limit, which could lead to annoying nested
  verical scroll regions.  (An inner region for the options inside a
  scrollable viewport.)

This is optional (but the default): You can still explicitly set COLSxROWS,
but "auto" uses a ResizeObserver sets the size based on window size.