[abashev]

The code and the result stood side by side, each half the width of the page, so a line of the formatter's 120 columns fitted in neither. They are now strips as wide as the page:

• the code on top, the Format button and the status under it, the result below: the bar moves between the boxes in playground/index.md, and the intro says "the upper box" instead of "on the left";
• each box is min(30vh, 20rem) high and can be pulled taller (resize: vertical), on every screen width, so the two-column breakpoints go;
• the code in them is 0.65rem, 13 px, instead of 0.75rem.

Looked at with the 21.0.2 module fetched by scripts/fetch-playground.sh, in Chrome at 1440 px and at 390 px: the module loads, the boxes take the width of the page, and nothing scrolls sideways on the phone.

zensical build --clean --strict reports no issues.