Most Likely Cause and Fix
Your markup makes use of the non-existent attribute
You should remove this attribute for your markup to be valid or consider using HTML5 custom data attributes.
What This Error Means
There is a defined set of attributes for each HTML element. You may choose to omit some but you cannot choose to add further attributes beyond those defined.
This error is simply saying that the attribute
is not defined for use on any element. It does not exist.
How To Fix It
Check your doctype
Make sure you're using the version of HTML you intended to use.
If you are transitioning from an older to a newer version of HTML, check that the attributes you are using are still valid.
The use of some attributes may become deprecated over time if they're found to not be as useful as intended. Some attributes may be removed entirely.
Check the document type declaration used in your markup. Fix this if not correct.
If you are using HTML5 custom data attributes
of the form
data-*, make sure you are using the
Simply remove the attribute
You have used the attribute
allowfullscreen on an element where the
element does not allow an attibute of that name.
The specific error message you encountered will refer to a line and column number. This will guide you to the exact point in your markup to which the error relates.
You should remove this attribute for your markup to be valid.
Use a HTML5 custom data attribute
Are you making up attributes merely as a means of storing data?
<img src="http://example.com/image.png" alt="Desc" allowfullscreen="jon" />
HTML5 custom data attributes are intended exactly for this purpose. Give it a go if relevant to you.
<img src="http://example.com/image.png" alt="Desc" data-allowfullscreen="jon" />