LSPW pointed out that headings in chapters which are nested in <div>s to the point where they become <h3> elements were not centred, whereas those which become <h2>s are centred by default. I added text-align: center to the h3 ruleset in the stylesheet, but then we discovered that reference items have <h3> headings, and those should be left-aligned, so I added another ruleset to set them back to left-aligned.