Why it matters
Horizontal overflow on a common phone width makes the whole page wobble sideways while scrolling and hides content.
How to fix it
Find the overflowing element in the evidence and constrain it (max-width: 100%, min-width: 0 on flex/grid children, overflow-wrap: anywhere).
More design rules
void design reports 19 rules in this category. Measured design heuristics from computed styles: font families, type scale, line length, palette and accent sprawl, spacing grid, radius sprawl, dark mode and the tells of generated UI.
gradient-textGradient-filled heading textpurple-gradientPurple/indigo gradient backgroundemoji-iconsEmoji used as iconsspacing-off-gridSpacing values are off the 4px gridradius-sprawlToo many different corner radiino-dark-modeNo dark mode supportcolor-scheme-missingcolor-scheme is not declaredselection-unstyled::selection is not styled