In plain language
If a user widens line, letter, word, and paragraph spacing (a common reading aid), no text may be cut off or overlap.
Who it affects
People with dyslexia or low vision who apply extra text spacing to read.
How to check
Apply a text-spacing bookmarklet (line 1.5, paragraph 2x, letter 0.12em, word 0.16em) and scan every page for clipped or overlapping text. Worked example — A recipe site's ingredient list: apply the text-spacing override and open tooltips/dropdowns: if they're sized for unspaced text and now truncate, it fails.
Common misconception
This isn't about your own line-height setting. It's about whether the page still works when a *user* overrides spacing with their own stylesheet, most sites have never tested that. Real example: on a used-car marketplace's listing filter, when a user increases line/letter/word spacing (a common low-vision aid), text gets cut off or overlaps other content.