The properties controlled by the ::highlight() pseudo cannot affect layout, so there's not going to be a full reflow when something change, but they can still cause visual changes, so we need to be clear about when a re-computation of styles and a repaint happen.

In addition to DOM changes that would cause a restyling anyway, the following can cause a need for restyling/repainting:

• adding or removing a HighlightRangeGroup to CSS.highlights
• adding or removing a range to a HighlightRangeGroup in CSS.highlights
• Changing the boundary points of a range in a HighlightRangeGroup in CSS.highlights
• Changing the priority of a HighlightRangeGroup in CSS.highlights
• Changing the style property of a HighlightRangeGroup in CSS.highlights

Are any or all of things things synchronously triggering a restyle/repaint? Probably not, but what should the spec say?