HTML email is a message built using HTML and CSS code to create a styled, visually formatted layout, including images, colors, and custom typography, rather than displaying as unformatted plain text.
Why Email HTML Isn't Web HTML
An HTML email looks similar to a webpage on the surface, but the underlying constraints are genuinely different, which is the first thing worth understanding about what is an HTML email at a technical level. Modern web development relies on flexible, current CSS layout techniques that browsers have largely standardized on.
Email clients never fully caught up to that standardization, some still render using outdated engines, which is why HTML email design still commonly relies on table-based layouts and inline CSS, techniques considered outdated for web design but still the most reliable way to get consistent rendering across email clients like Outlook, Gmail, and Apple Mail.
HTML Email vs Plain Text
HTML email vs plain text is a real choice, not just a style preference:
- HTML supports branding, product images, and a clear visual hierarchy that guides the eye toward a CTA.
- Plain text, stripped of any styling, can actually outperform HTML in specific contexts, cold outreach or highly personal-feeling messages, since it reads more like a genuine one-to-one email than a marketing campaign.
Most ecommerce marketing email defaults to HTML for the branding and product-display benefits, while transactional or personal-toned messages sometimes lean plain text deliberately.
Building One That Renders Reliably
HTML email best practices center on accounting for the fragmented rendering landscape rather than assuming any single email client’s behavior is universal:
- Test across major clients before sending, rather than checking just one.
- Keep file sizes reasonable, since large HTML emails can get clipped by clients like Gmail.
- Build with a single-column, mobile-first structure, given how much email now opens on a phone screen.
A poorly coded HTML email can render as a jumbled mess in one client while looking perfect in another, which is why HTML email remains one of the few areas of digital design where genuinely old techniques are still the professional standard.
The Multipart Structure Behind Every Well-Built Email
A properly built HTML email actually contains two versions bundled into one message: the styled HTML version and a plain text fallback, sent together in what’s called a multipart message.
Most subscribers never see the plain text version, since virtually every modern email client renders HTML by default, but it still matters for a few real reasons: some corporate email environments strip HTML for security, some accessibility tools rely on it, and its presence itself is a small positive signal to spam filters, since legitimate senders almost always include it while low-effort spam often doesn’t bother.
Skipping the plain text fallback entirely is a small technical oversight that can have an outsized effect on deliverability.
Related terms:
Adflipr’s drag-and-drop email builder generates clean HTML automatically, so campaigns render consistently without needing to hand-code table layouts.
