HTML Email Compatibility Review: test results

Verdicts

Date

2026-10-19

Strong · claude-sonnet-5-5 (Claude Code alias "sonnet")

Right on all 3 cases, read by hand. On the autumn-sale email it gave the flex wrapper and the background image with no colour behind the white heading, and left out the media-query style block. On the shipping notice it gave the head-only layout and the logo without alt, and left out the spacer with an empty alt. On the clean receipt it answered exactly No findings. My first version of one check wrongly rejected its mention of a media query inside a fix; the check was relaxed and the recorded answer re-evaluated without a new call.

Weak · claude-haiku-5-5 (Claude Code alias "haiku")

Right on all 3 cases, read by hand. It gave the same two lines for the autumn-sale email and the shipping notice, indented by four spaces, which the format check allows, and answered exactly No findings on the clean receipt, including its background image that has a dark colour set beside it.

Note

Light test: three cases (two planted, one clean), one run per model; no comparison without the skill. The three inputs were written by us; each planted one carries a safe look-alike and neither has a comment pointing at the defect. Facts come from the public pages named in the notes, read on 19 October 2026; there are no owner-measured facts.

What was not measured

• Models other than the two named above were not run.
• Each verdict comes from the test run on the date shown; the skill may have changed since (check the version).
• Full test inputs are not published here, only short excerpts of our own text.
• Results on your own texts, languages and domains can differ.