Most Likely Cause and Fix
In the context of
<Y X="W" />
X is not allowed to take the value
You should remove this attribute or set a valid value for your markup to be considered valid.
What This Error Means
Some element attributes can take pretty much any value, others can take only values of a certain type and others still can only take a value from a pre-defined set.
This error relates to the following usage in your markup:
The error is stating that, in this context, the attribute
Y is not allowed to take the value
The validator is giving the reason: Z
How To Fix It
The error relates to an invalid attribute value being used on a specific element.
How this might be fixed often depends on the element, attribute, attribute value and the context in which these are being used.
Pay attention to the additional information supplied by the HTML validator as this will often point you in the right direction: Z
Change the relevant attribute value to one that is valid for the context in which it is being used.