mixed content
Also called: mixed content warning, insecure content
Mixed content is when a page's HTML loads over secure HTTPS but pulls subresources (images, scripts, CSS, fonts) over insecure HTTP. Browsers auto-upgrade some types (images, audio, video) to HTTPS and block the rest (scripts, stylesheets, iframes) by default.
Upgradable vs blockable
The W3C spec, as implemented in Chrome, Firefox, and Safari, splits mixed content into two buckets. Upgradable content, meaning images, audio, and video referenced through a src attribute, gets its scheme quietly rewritten from http:// to https://. If the secure version does not exist, the request fails and the asset simply does not show. Blockable content, which is everything else (<script>, <link> stylesheets, <iframe>, web fonts, fetch() and XMLHttpRequest, srcset images), is refused outright.
The reason for the split is risk. A passive image fetched over HTTP can be swapped by an on-path attacker to show false information, but a script or stylesheet can rewrite the whole page or steal data, so browsers never let it through. You can force every request up to HTTPS, including the blockable kinds, with the Content Security Policy directive upgrade-insecure-requests set in a header or <meta> tag.
Common causes: a hardcoded http:// in a theme or plugin, an old CDN reference, or self-referencing absolute URLs left behind when an HTTP-era site was migrated to HTTPS without a find-and-replace. Chrome DevTools lists every instance in its Issues tab, which makes it a fast fix inside a broader Technical SEO cleanup.
How it affects your traffic
Mixed content is a technical-hygiene problem that leaks into rankings indirectly. When a browser blocks your active resources, say a stylesheet or a script that lays out the page, Googlebot renders the same broken version your visitors see, which can distort how the page is understood and hurt Core Web Vitals. Users also lose the full padlock, and a page that renders half-broken converts worse. Finding and clearing every insecure subresource is standard cleanup in a Technical SEO engagement.
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