Catalog > Size Charts: a chart is a table (pasted as text), a heading and a note, assigned to categories. A product shows the chart of its most specific category, or the one its Size Chart attribute names; a "Size guide" button above Add to Cart opens it in a native dialog. Saving a chart refreshes the cached product pages.
The size guide on a product page.
What it does
Charts by category, with an override per product
A native dialog, no JavaScript beyond the theme's
Pages stay in the full-page cache; a saved chart cleans them
