Email HTML checker: Gmail 102 KB clipping, alt text, contrast and links

Paste the HTML of an email. You get its size against the roughly 102 KB point where Gmail is reported to clip messages, plus every image without alt text, empty or shortened link, low-contrast text and leftover placeholder, and optionally whether your plain-text version carries the same links. It runs in your browser: the email is not uploaded anywhere.

How Gmail clipping works, and what is not known

Gmail is widely reported to cut off a message when its HTML is larger than about 102 KB and show "[Message clipped] View entire message". The count is of the code, not of the visible text, so repeated inline styles and pasted word-processor markup add up fast. Google does not publish the limit; the figure comes from consistent reports by email providers (for example ContactMonkey, Zoho and ActiveCampaign). This page flags an error at 102 KB and a warning from 80 KB. Read how to stay under it.

What this checker does and does not do

• It measures the file. It does not render the email in Outlook, Gmail or Apple Mail, so it cannot tell you how it looks there.
• Contrast uses inline colours only. Text and background colours set with style="..." or bgcolor are compared against WCAG 1.4.3 (4.5:1, or 3:1 for large text). Colours that come only from classes or media queries are not resolved, and where no background is set it assumes white and says so.
• Markup inside Outlook conditional comments is skipped, so ghost tables and VML buttons are not judged as web HTML.
• Spam-filter hints are heuristics. A phrase such as "act now" is a common trigger, not proof that a filter will block the message.

The same checker is open source: mailroom-check (MIT) runs on the command line with the same results. It is the free part of Mailroom Kit, fourteen finished transactional emails, each already passing it.