Loading page
About this tool
Audit email HTML for client compatibility, accessibility, spam risk and performance with a live preview and deterministic rule-based scoring.
The HTML Email Checker parses your email HTML and runs a fixed set of rules against the actual markup — DOCTYPE and table structure, inline CSS coverage, alt text and lang attributes, HTTPS images and links, message size against Gmail's clipping limit, and CSS features that Outlook, Gmail, Apple Mail or Yahoo silently drop — then scores six weighted categories into one grade from A+ to D. Every issue comes with why it matters, how to fix it and an example snippet, alongside a live preview and a 15-point pre-send checklist. Email developers and marketers get a deterministic audit: the same HTML always produces the same result, with no sending or account required.
Open HTML Email Checker on AltFTool — it loads instantly in your browser.
Paste your email's HTML into the HTML Source editor, or click Sample to load a demo, then click Check to run the audit.
Review the six category scores and the client compatibility matrix for Gmail, Outlook, Apple Mail and Yahoo Mail.
Work through the issues list — each one explains why it matters and how to fix it, with example code where relevant.
Export the full report as PDF or JSON, or download the cleaned HTML once every check passes.
Gmail, Outlook, Apple Mail and Yahoo each get their own score and a list of the exact features that break in that client.
Each issue states the reason, the fix and a copyable example — so 'low inline CSS coverage' comes with the percentage measured and what to inline.
Checks are string and DOM tests against your markup, so results are reproducible and you can see precisely which rule produced each penalty.
Each category starts at 100 and loses 18 points per error, 9 per warning and 3 per info issue; the six category scores are then combined with fixed weights — structure 20%, accessibility 20%, compatibility 20%, CSS 15%, performance 15% and responsive 10%. The total maps to a grade: 90+ is A+, 80+ A, 65+ B, 50+ C and below that D.
Around 102KB of message body, after which Gmail truncates and shows a '[Message clipped]' link — hiding everything below the cut, including footers and tracking pixels. The checker raises this as an error above 102KB and as a warning once you pass 80KB, so there is room to trim before it bites.
The checker flags display:flex and display:grid, position:fixed/absolute, @keyframes animations, CSS variables, transforms and gradient backgrounds, because at least one major client drops each of them — Grid fails in all four checked clients, and Outlook desktop ignores media queries because it renders with the Word engine. The reliable alternative is nested tables with inline CSS.
No — it analyses the HTML in your browser and renders a preview, so nothing is transmitted and no seed accounts are needed. That makes it fast for catching structural, accessibility and compatibility problems, but it is not a substitute for a real render test if pixel-level appearance in a specific client version matters.