No space between attributes.

An href attribute on an a element contains an invalid URL that has space characters in the domain.

The domain in a URL cannot contain space characters, for example the following are invalid:

<a href="http://my domain.com">link</a>
<a href="http://my%20domain.com">link</a>

End tags in HTML must not have any attributes.

According to the HTML specification, only start tags (also called opening tags) may contain attributes, while end tags must appear as a plain </tagname> with nothing else.

For example, this is invalid and will trigger the error:

<p>Welcome to the site.</p class="welcome">

Correct code removes the attribute from the end tag:

<p>Welcome to the site.</p>

If you need the element to have an attribute, only include it in the start tag:

<p class="welcome">Welcome to the site.</p>

Attributes for HTML elements must be defined like name="value". The name may be missing before the =.

Some examples:

<!-- Missing equal sign -->

<p name "value"></p>

<!-- Missing space before the attribute -->

<pname "value"></p>

A < character has been found when an attribute was expected instead. Check the syntax of the affected tag, it’s probably malformed and a < character inside has been interpreted as an attribute.

For example, this code might cause this issue:

<!-- Malformed img tag -->

<img src="photo.jpg" alt="smiling cat" < />

<!-- Fixed img tag -->

<img src="photo.jpg" alt="smiling cat" />

Attributes for HTML elements must be defined like name="value". A common issue is missing the =, or repeating the " like in these examples:

<!-- Missing equal sign -->

<p name "value"></p>

<!-- Repeated quotes -->

<p name="value""></p>