Validación HTML: Bad value “favicon” for attribute “type” on element “link”: Subtype missing.

Acerca de este problema HTML

MIME types (also called media types) follow a strict type/subtype structure as defined by IETF standards. For example, image/png has the type image and the subtype png. The value "favicon" doesn’t follow this format — it has no slash and no subtype — so the validator reports “Subtype missing.” This is a common mistake that happens when developers confuse the purpose of the icon (a favicon) with the format of the file (an image in a specific encoding).

While most browsers are forgiving and will still display the favicon even with an invalid type value, using incorrect MIME types can cause issues. Some browsers or tools may ignore the <link> element entirely if the type doesn’t match a recognized format. It also hurts standards compliance and makes your HTML less predictable across different environments.

The correct MIME type depends on the file format of your favicon:

.ico files: image/x-icon (or image/vnd.microsoft.icon)
.png files: image/png
.svg files: image/svg+xml
.gif files: image/gif

It’s also worth noting that the type attribute on <link rel="icon"> is entirely optional. If omitted, the browser will determine the file type from the server’s Content-Type header or by inspecting the file itself. Removing it is a perfectly valid fix.

Examples

❌ Invalid: using “favicon” as the type

<link rel="icon" href="/favicon.png" type="favicon">

The value "favicon" is not a valid MIME type, triggering the validation error.

✅ Fixed: using the correct MIME type for a PNG favicon

<link rel="icon" href="/favicon.png" type="image/png">

✅ Fixed: using the correct MIME type for an ICO favicon

<link rel="icon" href="/favicon.ico" type="image/x-icon">

✅ Fixed: using the correct MIME type for an SVG favicon

<link rel="icon" href="/favicon.svg" type="image/svg+xml">

✅ Fixed: omitting the type attribute entirely

<link rel="icon" href="/favicon.png">

Since the type attribute is optional for <link rel="icon">, removing it avoids the error and lets the browser detect the format automatically. This is often the simplest and most maintainable approach.