Sitemap Error: Unsupported format HTML
Open the sitemap in your favorite browser. If the sitemap loads with a 404 error, please follow the steps in this article. If the sitemap loads properly, compare the sitemap URL in your browser to the sitemap URL entered in Google Search Console. They should be identical.
Unsupported format. Your file appears to be an HTML page.
Typically this happens with video sitemaps when you’re running W3 Total Cache and you’ve ticked the setting to hide the sitemap from normal visitors. What W3TC does is cache the error which normal visitors get, if they want to view the video XML sitemap. This error is then also served to googlebot, because the logic to show the sitemap to googlebot won’t execute. The best way to fix this is to create an extra user agent group.
You can create this user agent group by going to the W3TC’s settings. Under Performance and then User Agent Groups there’s a button “Create a group”. Click this button and call your group “Google” or “Googlebot” and then in the user agents field you just enter “googlebot”. This will force W3 Total Cache to make a separate cache for Googlebot user-agents. Another option is to prevent W3TC from caching video-sitemap.xml entirely, but since that’s slower, we actually prefer the user-agent group.
What if it happens for non-video sitemaps?
The most common cause for this error on non-video sitemaps is entering the wrong sitemap URL or the sitemaps has an error.
Was the correct sitemap URL submitted?
The Yoast SEO sitemap URL to submit ends in sitemap_index.xml. If you submitted sitemap.xml, please remove it and submit the correct sitemap.
Does the sitemap error?
Open the sitemap in a browser. Remember, the Yoast SEO sitemap URL to submit ends in sitemap_index.xml. Does an error appear? If so, search our list of common sitemap errors for additional steps.