Reference

HTML Email Glossary

Definitions for 39 essential HTML email development terms — from email rendering and CSS inlining to the Outlook Word engine, dark mode, preheader text, and more. A quick reference for developers, designers, and email marketers.

@

@font-face: A CSS rule that allows custom fonts to be embedded in an email using a URL reference. Supported in Apple Mail, Thunderbird, and many modern email clients, but not in Gmail or Outlook for Windows. When `@font-face` is unsupported, the browser/client falls back to the next font in the `font-family` stack.

A

Alt Text: The `alt` attribute on `<img>` tags that provides a text description when images are blocked or fail to load. Many email clients block images by default — without alt text, image-only sections appear as empty boxes. Alt text is also required for accessibility and screen reader compatibility.
AMP for Email: An extension of Google's Accelerated Mobile Pages (AMP) technology that enables interactive content inside Gmail messages — such as forms, carousels, and live data. Supported only in Gmail and a small number of other clients. Requires sending a specially structured MIME part alongside the regular HTML and plain-text versions.

B

Bulletproof Button: An HTML email button technique that uses a `<table>` and `<td>` with background-color styling instead of a CSS `background-color` on a `<div>` or `<a>`, ensuring it renders correctly in all email clients including Outlook. Often also uses VML to achieve rounded corners in Outlook.

C

CSS Inlining: The process of converting CSS class rules into inline style attributes on individual HTML elements (e.g. `<p style="color: red;">` instead of `.red { color: red; }`). CSS inlining is required for maximum email client compatibility because many clients — especially Gmail — strip or ignore `<style>` blocks in certain contexts.
Conditional Comments: An HTML comment syntax specific to Outlook for Windows that allows developers to show or hide content only in Outlook. `` shows content only in Outlook. `content` hides content from Outlook. Used to inject VML workarounds and Outlook-specific table structures.
Compatibility Score: A per-client score indicating how well an HTML email's CSS and HTML will render in a given email client. MailViewr calculates this by checking each CSS property and HTML pattern against a database of 210+ known support rules per client. A score of 100% means no detected compatibility issues for that client.
CAN-SPAM: A US law (Controlling the Assault of Non-Solicited Pornography And Marketing Act) that sets rules for commercial email. Key requirements include: include a physical address, honor opt-out requests within 10 days, include a clear unsubscribe mechanism, and not use deceptive subject lines. Violations can result in fines up to $51,744 per email.

D

Dark Mode (Email): A display setting where email clients invert or override the email's colors to create a dark background with light text. Gmail on Android and iOS applies CIELAB L* lightness inversion algorithmically. Outlook mobile applies its own partially-documented overrides. Developers can target dark mode using `@media (prefers-color-scheme: dark)` in email clients that support it.
DOCTYPE Declaration: The `<!DOCTYPE html>` declaration at the very start of an HTML document that tells the browser or email client which version of HTML to expect. Its absence causes some email clients to render in quirks mode with inconsistent results. Best practice: always include `<!DOCTYPE html>` as the first line of your email HTML.

E

Email Rendering: How an email client interprets and displays the HTML and CSS in an email. Email rendering varies significantly across clients — Gmail, Outlook, and Apple Mail each have different rendering engines with different CSS support. "Email rendering" refers to the visual output the recipient sees.
Email Client: The application or web service a recipient uses to read email. Examples include Gmail (webmail and app), Outlook (desktop and mobile), Apple Mail (macOS and iOS), Yahoo Mail, and Thunderbird. Each email client has its own HTML/CSS rendering engine, meaning the same email can look different across clients.
Email Service Provider (ESP): A platform used to send bulk or transactional emails. Examples include Mailchimp, SendGrid, Klaviyo, Brevo, and HubSpot. ESPs handle deliverability, unsubscribe management, and analytics. Each ESP uses its own templating syntax: Mailchimp uses merge tags like `*|FNAME|*`, while SendGrid and Klaviyo use Handlebars or Jinja2.
Email-Safe Fonts: Fonts that are pre-installed on the operating systems of most email recipients and therefore render predictably without loading from an external URL. The core email-safe fonts are Arial, Helvetica, Georgia, Times New Roman, Verdana, Tahoma, Trebuchet MS, and Courier New. A font stack (e.g. `font-family: Arial, Helvetica, sans-serif`) ensures fallback rendering.
External Stylesheet: A CSS file linked with `<link rel="stylesheet" href="...">` in the email HTML. External stylesheets are blocked or stripped by virtually all email clients — Gmail, Outlook, Apple Mail, and Yahoo. All CSS must be embedded in a `<style>` block or applied as inline styles directly on HTML elements.
Email Accessibility: Designing emails to be usable by people with disabilities, including those using screen readers, high-contrast modes, or keyboard navigation. Key practices: use semantic HTML, include alt text on all images, ensure sufficient color contrast, set `role="presentation"` on layout tables, and use a logical heading structure. Many email clients strip ARIA attributes.

F

Fluid Layout: An HTML email layout where widths are set as percentages rather than fixed pixel values, allowing the email to resize fluidly on any screen width. Fluid layouts are an alternative to media-query-based responsive design and are useful for email clients that do not support media queries (like some versions of Outlook). Also called "liquid layout".

G

Gmail Clipping: Gmail truncates emails with HTML source code exceeding 102KB, replacing the rest with a "View entire message" link. This hides the bottom of the email including the unsubscribe link, breaking CTAs and compliance. Best practice: keep email HTML under 102KB, often achieved by removing comments and whitespace.
Gmail Style Stripping: Gmail — in certain contexts such as emails sent via some APIs or non-authenticated senders — strips the entire `<head>` and `<style>` block, leaving only inline styles active. This makes CSS inlining critical for Gmail compatibility. Gmail also munges class names (e.g. `.button` becomes `.m_1234button`) which can break CSS selectors.
GDPR (Email): The EU General Data Protection Regulation as it applies to email marketing. GDPR requires explicit consent before sending marketing emails to EU residents, a clear description of how data will be used, an easy unsubscribe mechanism, and the right to erasure upon request. Transactional emails to existing customers may be exempt under legitimate interest.

H

HTML Email: An email that uses HTML and CSS markup to create formatted, visual layouts — as opposed to a plain-text email. HTML emails can include images, buttons, columns, fonts, and branded styles. The challenge is that HTML email rendering differs significantly across email clients, requiring special coding techniques.
HTML Email Checker: A tool that analyzes HTML email code for compatibility issues, rendering problems, and best-practice violations. MailViewr's built-in compatibility checker scans both `<style>` block CSS and inline `style=""` attributes, detecting unsupported CSS properties per email client and giving a compatibility score.

I

Inline Styles: CSS styles applied directly on an HTML element via the `style` attribute (e.g. `<p style="color: red; font-size: 16px;">`). Inline styles are the most reliable form of CSS in HTML email because they survive style-stripping by email clients that remove `<head>` or `<style>` blocks. CSS inlining tools automatically convert class-based CSS to inline styles.
Image Blocking: A setting in many corporate email clients and Outlook by default that prevents images from loading in emails until the user explicitly allows them. Emails must be readable and functional with images blocked — this requires alt text on all images and layouts that do not depend entirely on images to convey meaning.

M

Media Query (Email): A CSS rule that applies styles conditionally based on device characteristics such as screen width or user preferences. In HTML email, `@media` queries are used to make emails responsive on mobile. Support varies: modern email clients support them, but Outlook for Windows ignores all media queries.
MIME Multipart: A standard email format that bundles multiple versions of the same email content in a single message — typically a `text/plain` and a `text/html` version. Email clients choose the version they prefer to display. Well-configured ESPs automatically generate the plain text version from the HTML. Adding an `application/json` AMP part is also done via MIME multipart.

O

Outlook Word Rendering Engine: Outlook for Windows (2007–2021+) uses Microsoft Word's rendering engine to display HTML email, rather than a browser engine. This causes Outlook to lack support for modern CSS properties including flexbox, CSS grid, border-radius, background-image on divs, and many others. Developers use conditional comments and VML as Outlook-specific workarounds.

P

Preheader Text: The short summary text that appears next to or below the email subject line in most email clients' inbox views. It is typically set as hidden text at the very top of the email HTML (e.g. `<span style="display:none;">Preview this email...</span>`). A good preheader improves open rates by giving context before the email is opened.
Plain Text Email: An email containing only unformatted text, with no HTML markup. Plain text emails are highly deliverable, render identically across all clients, and are often preferred for personal or transactional messages. Most ESPs send a multipart message with both HTML and plain text versions so email clients that cannot render HTML fall back to plain text.

R

Responsive Email: An HTML email that adapts its layout to different screen sizes — typically showing a single-column layout on mobile and a multi-column layout on desktop. Achieved using `@media` queries, fluid-width layouts, or hybrid/spongy layout techniques. Not all email clients support media queries, so fluid-width or "mobile-first" approaches are often more reliable.
Retina / HiDPI Images: Email images displayed on high-density screens (like Apple Retina displays) appear blurry if the image resolution is too low. Best practice: use images at 2× the display size (e.g. a 300px wide image should be 600px wide at 1px per device pixel ratio) and set the `width` attribute to the intended display size.

S

Sender Reputation: A score maintained by email providers (Gmail, Outlook, Yahoo) that reflects the trustworthiness of an email sender's domain and IP address. High spam complaint rates, high bounce rates, and sending to unverified addresses lower sender reputation. Low reputation causes emails to be routed to spam folders or blocked entirely.
SPF / DKIM / DMARC: Email authentication protocols that verify a sender's identity and prevent spoofing. SPF (Sender Policy Framework) specifies which servers are allowed to send email for a domain. DKIM (DomainKeys Identified Mail) adds a cryptographic signature to emails. DMARC (Domain-based Message Authentication) specifies what to do with emails that fail SPF/DKIM checks. All three are required for reliable email delivery to Gmail and Yahoo.

T

Table-Based Layout: A method of structuring HTML email using `<table>`, `<tr>`, and `<td>` elements for layout instead of CSS flexbox or grid. Table-based layout is considered the most reliable approach for email because all email clients — including Outlook for Windows — support table layout. Modern CSS layout methods (flexbox, grid) are not reliably supported across email clients.
Transactional Email: An automated email sent in response to a specific user action or system event — such as order confirmations, password resets, shipping notifications, and account alerts. Transactional emails are typically sent one-to-one (per user event) rather than in bulk. They are exempt from some anti-spam regulations but should still include an unsubscribe mechanism for marketing transactional content.

U

Unsubscribe Link: A link in a marketing email that allows recipients to opt out of future emails. Required by CAN-SPAM, GDPR, and similar regulations. Gmail and Yahoo enforce a one-click unsubscribe requirement for bulk senders. Missing an unsubscribe link increases spam complaints, damages sender reputation, and is legally required for commercial email.

V

VML (Vector Markup Language): A Microsoft markup language used to draw shapes and backgrounds in Outlook for Windows, which does not support CSS backgrounds on non-table elements. VML is used to create CSS `background-image` and `border-radius` workarounds in Outlook. It is wrapped in conditional comments so only Outlook renders it: ``.
Viewport Meta Tag: The `<meta name="viewport" content="width=device-width, initial-scale=1">` tag in the `<head>` of an HTML email. Without it, mobile email clients may render the email at the desktop width and then scale it down, making text tiny. Including the viewport meta tag enables proper mobile rendering.

W

Web Fonts in Email: Custom fonts loaded via `@font-face` or Google Fonts `@import` in HTML email. Supported in Apple Mail (macOS and iOS), Thunderbird, and some modern webmail clients. Not supported in Gmail or Outlook for Windows. Always define a fallback font-family stack so email clients that do not support web fonts render the email using an email-safe font.

Test Your HTML Email

Preview how your email renders across Gmail, Outlook, Apple Mail, and more — instantly, with no signup.

Open Email Previewer