# Email Design - Constraint Catalogue This reference is the **email-constraint catalogue**: the hard email-client rules an email template MUST honor. Email is a *constrained* HTML/CSS surface - the modern HTML/CSS the web executor emits (flexbox, grid, `` block is flagged as the primary styling mechanism when, after removing `@media { … }` groups and `@font-face` from the block, residual CSS rules remain **or** the block exceeds a generous size threshold. A small dark-mode/responsive `@media`-only block (per EM-STYLE-04) is therefore tolerated and does NOT trip the check. - **EM-DARK-01 is decoupled from `