A custom HTML email block should solve a specific problem that the standard components do not cover. It should also remain usable when a product name grows, an image is missing, or the email is opened on a phone. Define the smallest component you need, test it outside the editor, and reuse the version that survives those conditions.
Define the smallest required change
Write down what the block must communicate and why the existing layout cannot do it well. Keep the markup focused on that job. Avoid importing a full web-page design with scripts, interactive dependencies, or styling that email apps may not support.
Sendvio supports HTML email blocks. Keep any custom code understandable to the person who will maintain the template next month. Use clear structure, meaningful text, and appropriate image alternatives rather than hiding the message inside decorative markup.
Define a small component contract
Write the block's purpose, required content, optional content, maximum realistic text length, links, image behaviour, and mobile order. For example, a two-option comparison block might require a product name, one distinguishing feature, price context, and an action for each option. The contract should explain what must remain together when the layout stacks.
Ask whether standard blocks can meet that need with a simpler arrangement. Custom HTML is justified when it solves a clear communication problem, not merely when it allows a more elaborate composition. Every additional structural rule creates something the next editor must understand and test.
Keep the custom block self-contained. Avoid broad styles that unexpectedly change neighbouring components, and do not depend on scripts or website-style interactions for the main message. If the experience requires complex selection or calculation, use the email to explain the task and link to an appropriate page that can support it.
Check behavior outside the editor
Email rendering differs from ordinary website rendering. Test the actual sent message across relevant apps, including a narrow screen and images unavailable. Verify links, reading order, text resizing, and how the block behaves beside standard template components.
Avoid fixed heights around variable content and long unbreakable strings. A customer name or translated heading can expose a failure that the short sample text hides. Keep a simpler fallback design available if the custom block cannot behave reliably.
Test content changes, not only the original sample
Replace the shortest product name with the longest realistic one. Remove an optional image, add a translated heading, and use a longer price or button label. Check whether the structure grows naturally or leaves clipped content and empty fixed-height areas. These tests reveal whether the block is reusable or merely fitted to one example.
Inspect the actual sent message in relevant email environments. Verify that the reading order remains meaningful, that image alternatives fit the context, and that every destination works. Where layout tables are used, the implementer should ensure they do not misrepresent the content's semantic structure to assistive technology.
Reuse only the tested version
Save the approved block with a short note describing its purpose and limitations. When changing content, distinguish a routine text edit from a structural change that needs another rendering review.
Do not add tracking scripts or executable code to make an email behave like a website. Keep the action on a suitable destination page when interaction exceeds what an email can reliably do. A small dependable block serves the customer better than a sophisticated component that works only in the creator's preview.
Keep the approved block in versioned template notes with a clear owner, its supported use, and known limitations. A future text edit may be low risk, while changing column structure or adding new conditional content deserves another rendering review. Make that distinction visible so the team neither retests everything unnecessarily nor treats a redesign as a routine copy change.
Prepare a simpler fallback if the component cannot behave reliably. A single-column comparison that customers can read is more useful than a complex layout that fails in a common environment. The acceptance test is practical: accurate content, intact grouping, readable text, accessible actions, and a maintainable structure that the next campaign owner can safely reuse.