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Troubleshooting Common WebP Express 'Alter HTML' Issues

26 threads Sep 10, 2025 Plugin → Webp express

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The 'Alter HTML' feature in the WebP Express plugin is a powerful tool for automatically serving WebP images to compatible browsers. However, as a third-party support resource, BugWP.com has observed that this feature can sometimes conflict with other plugins, themes, or specific website functionalities. This guide outlines the most common problems users encounter with 'Alter HTML' and provides practical solutions based on community experiences.

Common Problems and Their Solutions

1. XML Sitemap Corruption

Problem: When 'Alter HTML' is active, you may find a comment like <!-- Alter HTML was skipped because the HTML is too big to process! (limit is set to 600000 bytes) --> at the top of your XML sitemap, breaking its validation.

Solution: The most common fix is to change the replacement method. Navigate to Alter HTML -> HOW TO REPLACE and select 'Use content filtering hooks (the_content, the_excerpt, etc)'. This method is less likely to process non-page content like sitemaps. For Yoast SEO specifically, some users have successfully used a custom code snippet in their theme's functions.php file to prevent this issue.

2. Conflicts with Page Caching Plugins

Problem: Enabling 'Alter HTML' can cause issues with caching plugins like Cache Enabler or WP Rocket, preventing them from creating separate cached versions for WebP and standard images.

Solution: Compatibility can often be restored by switching the replacement method. Try using 'Replace image URLs' instead of 'Replace tags with <picture> tags' under the 'HOW TO REPLACE' setting. Additionally, ensure the 'Where to replace' option is set to 'Use content filtering hooks'.

3. Admin or AJAX Functionality Breaks

Problem: The media library, AJAX calls (e.g., product filters), or custom PHP download scripts may stop working correctly because the output buffering interferes with their processes.

Solution: A known workaround is to disable 'Alter HTML' for the admin area. This can be tested by adding a simple code check to the plugin's `AlterHtmlInit.php` file. More permanently, if your site relies heavily on AJAX-loaded content, the 'Use content filtering hooks' method may provide better compatibility than full page output buffering.

4. Styling or Layout Issues

Problem: The HTML alteration can sometimes break a theme's styling or layout, making images appear tiny or incorrectly aligned. This is often due to conflicts with a theme's framework (like Redux) or existing <picture> tags.

Solution: First, try switching between the two 'HOW TO REPLACE' methods to see if one works better with your theme. If your theme already uses <picture> tags, the 'Replace image URLs' method is generally recommended to avoid nested tags. If the issue is tied to a specific framework, you may need to reach out to your theme's support for guidance.

5. Social Media Scrapers Not Seeing Images

Problem: Social media platforms like LinkedIn or Facebook may fail to scrape images when .htaccess rules are used for redirection, sometimes showing a grey box instead.

Solution: This is a known challenge. While 'Alter HTML' can help by providing standard image URLs in the HTML, the most reliable method for social scrapers is often to configure your .htaccess rules to serve original images to their user agents. The WebP Express documentation may have guidance on setting up such user-agent detection.

General Troubleshooting Steps

  • Clear All Caches: After changing any WebP Express setting, clear your site's cache, object cache, and CDN cache.
  • Toggle the Feature: If 'Alter HTML' unexpectedly disables itself, ensure your plugin is updated to the latest version, as a bug causing this was reportedly fixed.
  • Check Settings: If images aren't being replaced, verify that your CDN URL is correctly entered in the settings if you are using one.
  • Test Different Methods: The two primary methods—'Picture tags' and 'Replace URLs'—behave differently. If one causes an issue, test with the other.

The 'Alter HTML' feature is powerful but operates by processing your site's HTML, which can sometimes lead to conflicts. The solutions listed here, gathered from community reports, resolve the majority of these issues. For more specific or persistent problems, consulting the official WebP Express support forums or your theme/plugin developers may be necessary.

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