CSS (Cascading Style Sheets) is the language that controls how HTML is rendered — colour, layout, typography, spacing, responsive behaviour. On the web, CSS lives in external stylesheets that browsers download alongside the page. In email, CSS is constrained: many email clients strip external styles, ignore modern selectors, and require styles to be inlined directly on each HTML element.
How CSS works in practice for email. Mature email platforms convert your editor styles into inline CSS at send time. Properties known to be unsupported in Outlook or older clients fall back to tables and width attributes. Dark-mode adaptations rely on the small number of properties Gmail and Apple Mail respect. The result is a hybrid output: a modern HTML document with inlined styles and graceful fallbacks, designed to render in clients ranging from Outlook 2007 to iOS 17.
Why CSS matters for senders. The CSS you write decides whether your campaign renders cleanly across the inbox landscape — or breaks in the one client a key segment of your list uses. Letting your sending platform handle CSS inlining, fallbacks and dark-mode adaptation is the difference between a campaign that works everywhere and one that quietly looks wrong for 20% of recipients.
Mailpro and CSS
Email CSS, inlined automatically
Mailpro inlines your CSS at send time, so the styles you wrote in the editor survive Gmail clipping, Outlook stripping and mobile rendering — without you touching the source.