Runtime BiDi Structural Verification Catches Right-to-Left Layout Defects that Accessibility, Actionability, and Logical-Property Linting Miss

arXiv:2610.32934v1 Announce Type: new Abstract: Bidirectional (right-to-left, RTL) layout defects arise when interfaces authored with physical directional CSS (margin-left, border-left, text-align:left) are rendered under dir="rtl" without being mirrored. Accessibility engines (axe-core), actionability engines (Playwright), and even Stylelint's current core logical-property rule fail to localise these defects. We present runtime structural invariants -- a zero-golden mode over computed style and geometry, and a bidi-conservative differential mode -- that catch them in a headless browser. On a held-out benchmark of 14 real RTL bugs reconstructed from dated fix commits in Bootstrap and Material Components Web, the detector recovers 13/14 (2 via zero-golden, 11 via the differential mode), with no false alarm on any patched version; on the actual committed historical bootstrap.rtl.css it catches 1 of 2 attempted. It reports 0 false positives on 16 negative LTR-island cases and, in both modes, on 43 unmodified real components across four design systems and three RTL architectures (rule-of-three 95% upper bound