How to Fix AMP CSS Validation Error Caused by [sizes=”auto” i] in WordPress 6.7+

If you are using the official AMP plugin with WordPress 6.7 or newer, you may encounter this error when validating your AMP pages:

The attribute ‘style amp-custom’ in tag ‘style amp-custom’ is set to the invalid value…

The issue stems from invalid CSS selectors automatically inserted by WordPress into AMP pages. Specifically, selectors like:

Advertisement

‹

EVENT

Saksham Bharat 2026

A multi-stakeholder dialogue on skilling gap in Cybersecurity, Data Resilience and AI — and the roadmap to a Saksham Bharat.

📅 Tue, 26 May 2026

📍 Mumbai, Bengaluru, Delhi

Register Now →

EVENT

VeeamON 2026 Tour India - Mumbai

A VeeamON 2026 India Leadership Series Mumbai for senior public sector and government technology leaders.

📅 Tue, 26 May 2026

📍 Grand Hyatt, BKC

Register Now →

EVENT

Cyber Surakshit Uttar Pradesh

Find out strategies, frameworks and solutions for building a resilient and secure digital ecosystem across Uttar Pradesh.

📅 Thu, 28 May 2026

📍 Lucknow

Register Now →

EVENT

VeeamON 2026 Tour India - Bengaluru

A VeeamON 2026 India Leadership Series Bengaluru for senior public sector and government technology leaders.

📅 Wed, 03 Jun 2026

📍 JW Marriott Hotel

Register Now →

EVENT

VeeamON 2026 Tour India - Delhi

A VeeamON 2026 India Leadership Series Delhi for senior public sector and government technology leaders.

📅 Fri, 19 Jun 2026

📍 Shangri-La Eros

Register Now →

EVENT

Infosec Reimagined

Infosec Reimagined 2026 is the premier information security summit where top leaders—CISOs, CROs, CIOs, CTOs and risk executives—converge to redefine cyber resilience.

📅 Fri, 26 Jun 2026

📍 New Delhi

Register Now →

EVENT

Digital Senate

Digital Senate is a premier conference uniting government leaders, technologists and innovators to share ideas, success stories and strategies on digital governance, public sector transformation, cybersecurity and emerging technologies in India.

📅 Thu, 30 Jul 2026

📍 New Delhi

Register Now →

EVENT

CIO Prism

CIO Prism unites forward-thinking technology leaders to exchange transformative insights, shape digital strategies, and foster innovation, empowering enterprises to excel in an era of rapid technological change.

📅 Fri, 18 Sep 2026

📍 Goa

Register Now →

›

The issue stems from invalid CSS selectors automatically inserted by WordPress into AMP pages. Specifically, selectors like:

<style amp-custom>
img:is([sizes=”auto” i], [sizes^=”auto,” i]) {
contain-intrinsic-size: 3000px 1500px;
}
</style>

The i flag inside attribute selectors is not valid AMP-compliant CSS, and causes your AMP pages to fail validation.

Let’s walk through all the ways to fix it, from safe and recommended to advanced and temporary.

✅ Recommended Fixes

1. Create a Custom Plugin to Disable Auto-Sizes in AMP

This is the safest and most future-proof method. You’ll disable the auto-sizes feature only for AMP requests, preventing the faulty CSS from being injected.

🔧 Steps:

1. Go to your website files via FTP, cPanel, or your local WordPress install.

   Advertisement

2. Navigate to:

   wp-content/plugins/


3. Create a new file:

   disable-amp-auto-sizes.php


4. Open it in a code editor and paste:

   <?php
/*
Plugin Name: Disable AMP Auto Sizes
Description: Disables auto sizes attribute on AMP pages to fix invalid CSS in WordPress 6.7+
Version: 1.0
*/function amp_disable_img_auto_sizes() {
   if ( function_exists( ‘amp_is_request’ ) && amp_is_request() ) {
   return false;
   }
   return true;
   }
   add_filter( ‘wp_img_tag_add_auto_sizes’, ‘amp_disable_img_auto_sizes’ );

5. Save the file.

6. In your WordPress dashboard, go to Plugins > Installed Plugins and activate “Disable AMP Auto Sizes”.

7. Go to AMP > Tools > Purge AMP Cache to refresh old AMP pages.

8. Clear any page caching plugin or server cache (like LiteSpeed, WP Rocket, or Cloudflare).

✅ You’re done. AMP validation should now pass.

2. Use a Child Theme’s functions.php File

If you prefer not to create a plugin, you can add the same filter to your child theme’s functions.php file.

⚠️ Do not modify your main theme’s functions.php — changes will be lost during updates.

🔧 Steps:

1. Make sure a child theme is active. If not, install one. For Newspaper theme users, download the tagDiv child theme here.

2. Open:

   wp-content/themes/tagdiv-child/functions.php


3. Add this code at the bottom:

   function amp_disable_img_auto_sizes() {
if ( function_exists( 'amp_is_request' ) && amp_is_request() ) {
return false;
}
return true;
}
add_filter( 'wp_img_tag_add_auto_sizes', 'amp_disable_img_auto_sizes' );


4. Save and clear cache.

✅ This achieves the same effect but uses your theme instead of a plugin.

🛠️ Advanced (Not Recommended): Edit WordPress Core File

⚠️ Only for emergency use or experimentation. Not safe for production or future updates.

This method involves commenting out the line of code in WordPress core that inserts the invalid CSS.

🔎 How to Find the Code:

1. Go to:

   wp-includes/media.php


2. Search for this line (introduced in WordPress 6.7.1):

   $styles .= 'img:is([sizes="auto" i],[sizes^="auto," i]){contain-intrinsic-size:3000px 1500px}';


3. Simply comment it out:

   // $styles .= 'img:is([sizes="auto" i],[sizes^="auto," i]){contain-intrinsic-size:3000px 1500px}';


4. Save the file, clear cache and test again.

✅ The CSS will no longer be injected.

❌ But: WordPress updates will overwrite this file, so this fix won’t last unless you repeat it after every update.

💬 Frequently Asked Questions

Q: Why is this CSS even there?
A: WordPress 6.7+ introduced automatic handling of <img sizes="auto"> to improve responsive images. The CSS is valid for normal browsers, but AMP does not support the i flag in selectors.

Q: Will disabling auto-sizes break images?
A: No, AMP already optimises image loading. Removing auto-sizes will not noticeably affect layout or performance.

Q: I’m using the Newspaper theme. Which functions.php should I use?
A: Only use the one inside your child theme. If you’re unsure, install the tagDiv Child Theme and activate it.

🧪 Test Your Fix

Use either of the following tools to re-test your AMP page:

• Google AMP Test Tool

• AMP Page Experience Checker

🎉 Finally

The [sizes="auto" i] issue is a result of an update in WordPress that doesn’t fully account for AMP CSS limitations. While WordPress may resolve this in future versions, the above workarounds will help you maintain a clean, validated AMP setup in the meantime.

The best and safest way is to use a small custom plugin that disables auto-sizes only for AMP pages — no risk to your theme or core.

Let me know if you want a downloadable plugin ZIP version or if you’d like to publish this as a post with custom screenshots or code blocks.