Top 10 HTML Errors in Spain

An invalid attribute has been found on an element. Check the affected tag to ensure attributes are well-formed, and if they are you can consider using custom data attributes.

The id attribute is used to identify a single element within a document, and is required to be unique. Check the document for repeated IDs.

An end tag for X has been found that does not correspond to a previous open tag. This usually happens when you close the same tag twice.

The element X is not allowed as a child element of Y. For example, a <ul> element cannot have a <div> child element.

Check the HTMLImageElement.srcset guide to learn about the correct usage of the srcset and sizes attributes.

A CSS definition for an X property could not be understood by the parser. Check it to ensure that it’s well formed and that it contains an appropriate value.

The element X requires an attribute Y, but it’s not present. Check that element and add the required attribute, like in <X Y="value">.

<img> tags used to display images require the attribute src to indicate the source of the image, for example <img src="/img/photo.jpg" />.

<img> tags, used to include images on a document, require an alt attribute to describe the contents of the image. This is essential for users that cannot see the image (like screen reader users), or as an alternate text when the image cannot be displayed.

The @container at-rule is a recent addition to CSS which isn’t officially a standard rule yet, although widely supported by major browsers.