The Custom HTML block lets you add your own code to emails in Omnisend's Email Builder. Use it to embed third-party widgets, apply custom styling, or insert advanced content elements like countdown timers.

Two ways to use custom HTML in Omnisend:

This article covers the HTML block workflow for Email Campaigns and Automation Workflows.

Before adding HTML, check:

1. In Email Builder, open the Add Elements menu and drag the HTML block into your email content.

2. Click the HTML block to open the HTML editor on the right sidebar.

3. Paste or write your HTML code in the editor.

4. Click Save.

The HTML block lets you add custom code like countdown timers, tables, and widgets. Use inline CSS for styling and test your email before sending. 

To ensure your emails are compatible with major email clients, the HTML block supports only a limited set of HTML tags, attributes, and CSS styles.

<a>, <p>, <br>, <span>, <hr>, <ul>, <li>, <ol>, <u>, <strong>, <em>, <s>, <sub>, <sup>, <pre>, <h1>, <h2>, <h3>, <img>, <table>, <tr>, <td>

View full list with attributes:

Your code contains unsupported tags or encoding issues.

To fix:

Unsupported tags that cause errors:
​ <iframe>, <script>, <div>, <font>, <video>, <audio>, <object>, <embed>

Blank previews usually mean image URLs are missing https://, or you've removed required wrapper tags.

To fix:

No. Omnisend's Email Builder is visual-first and doesn't provide a raw HTML editor for the entire email.

You can only edit HTML within individual Custom HTML blocks.

To edit full email HTML:

Email clients like Gmail, Outlook, and Apple Mail override CSS in dark mode regardless of !important declarations.

Recommendation:
Design your email to look good in both light and dark modes. Test using Preview → Dark Mode toggle before sending.

Omnisend doesn't highlight the exact line causing the error.

To fix:

You can add countdown timers to Email Campaigns or Automation Workflows using the Custom HTML block.

Choose your setup:

Step 1: Drag and drop the HTML block into your email template.

Step 2: Create a countdown timer:

Step 3: Paste the code into the HTML block in Omnisend and click Save. 

To center the timer:

To make the timer dynamic (count from send time):

Add [[ current_date | date: "%s" | plus: 86400 ]] to the image URL.

This function converts the current date/time to seconds and adds 86400 seconds (1 day) to the send time.

Example:

You can change 86400 to any number of seconds. For example:

For Automation Workflows, you need a dynamic timer that calculates the expiration date each time the email is sent.

Example use case: Add a dynamic countdown timer that expires when a discount code expires (2 days after email is sent).

Let's add our timer based on a specific use case: imagine you want a dynamic timer that expires at the end of the sale (using a discount item).

Step 1. Create your automation in Omnisend → Open Email Builder and add a discount block for your sale. Set the expiration date for your discount, which will be the same as the expiration date for your timer.

Step 2. Add an HTML code here with a countdown timer showing when the discount code expires. Copy the Year-Month-Day and Timezone Offset HTML from Nifty images and add it to the HTML Item in Omnisend.

Now you need to change the date format to the Omnisend date format. Find the date in your code. You will see there is a dash between each personalization tag, which you will replace with Omnisend dynamic date tags.

Remove the date part from the code and insert Omnisend personalization tags:[[current_date | date:'%Y']]-[[current_date | date:'%m']]-[[current_date | date:'%d']].

Once replaced, the date will show zeros, but when the email is sent out, an actual date will be shown here because we added personalization tags.

Step 3. Adjust the timezone. In personalization, you will see a time zone set to Greenwich time. You need to change it to the timezone of your account. As shown in the screenshot below, change "T00:00:00-07" to "T00:00:00+2," etc., based on your timezone.

Step 4. Your timer is set to today's date, but you also want it to count dynamically till the end of your sale (discount expiry is set to 2 days).

For example, if your discount code expires in 2 days, you need your timer to run dynamically for 72 (it doesn't count today, so you need to add 24h to 2 days).

For this, go to Niftyimages, copy this code, return to the HTML item in Omnisend, and place it right after the format tag. Please note that the number of hours should be adjusted based on your sales duration. If your discount expires in 10 days, you must put 264 (240h+24h).

Here's an example of code pasted in Omnisend:

That's it; you've set the timer to run till the end of your promotion. Please remember that you can only test automation by triggering it yourself.

If you designed your email in Canva and want to import it as a full template: