HTML Guide for imagesrcset

A link element is invalid unless it contains at least one of the href or imagesrcset attributes.

The link element is used to define relationships between the current document and external resources, most commonly stylesheets. According to the HTML standard, a link must have either an href attribute, which points to the resource, or an imagesrcset attribute for responsive icons. If both are missing, the validator throws an error. This ensures that the link actually references a resource, otherwise it serves no functional purpose.

Correct usage:

With an href (for stylesheets, icons, etc.)

<link rel="stylesheet" href="styles.css">

With imagesrcset (for responsive icons)

<link rel="preload" as="image" type="image/png" imagesrcset="icon-1x.png 1x, icon-2x.png 2x">