| E721 | Unknown at-rule | The at-rule is not recognised as valid for the CSS Level used for the validation. |
| E722 | The @charset rule is not allowed here | The @charset rule can only be used in a separate CSS document and not in (X)HTML documents. |
| E723 | The @charset rule must be the first line in the document | The exact 10 characters '@charset "' must be the first thing in the document. Anything, even white space, prior to this will trigger this error.See http://www.w3.org/TR/CSS2/syndata.html#charset (displayed in new window). |
| E724 | The character set is not recognised | The character set specified is not recognised. This may be because it is misspelled. |
| E725 | The @import rule does not precede all other rules | Any @import rules must precede all other rules, except the @charset rule.See http://www.w3.org/TR/CSS2/cascade.html#at-import (displayed in new window). |
| E726 | The at-rule used is not formatted correctly | The at-rule used does not follow the accepted format for this rule.Please consult the relevant documentation for details. |
| E727 | Invalid characters found in URI | URIs can only contain a limited set of US-ASCII characters. See http://www.ietf.org/rfc/rfc2396.txt (displayed in new window). |
| E728 | An item is not allowed here | An at-rule, property or other item was used which is not allowed in this context. |
| E729 | Invalid @page selector used | The @page rule can only be used with a restricted number of selectors. In this case the selector used was not recognised as valid for the @page rule. |
| E730 | Missing font family | With the @font-feature-values rule, one or more font family names should be provided. |
| E731 | One or more of the listed font families is badly formatted | One or more of the font family names listed do not meet the requirements of the specification. |
| E732 | Missing feature value list | With font variant value definitions, a list of feature values must be provided. |
| E733 | Invalid feature value list | The list of feature values does not meet the requirements of the specification. |
| E734 | One or more required font descriptors are missing | The @font-face at-rule must have both a 'font-family' and a 'src' descriptor. |
| E735 | The @namespace rule does not precede all other rules | Any @namespace rules must precede all other rules, except the @charset and @import rules. See http://www.w3.org/TR/css3-namespace/#syntax (displayed in new window). |
| E736 | The namespace NAMESPACE has not been not declared | A @namespace rule declaring the namespace used in the selector was not found. |
| E737 | A counter-style type descriptor is required | MISSING_FEATURE_VALUE_LIST |
| E738 | The counter-style name has an invalid name | The @counter-style at-rule cannot have any of these names: 'decimal', 'default', 'hanging', 'inherit', 'initial', 'inline', 'inside', 'none', or 'outside'. |
| E755 | Pseudo-elements can only appear at the end of a selector | Pseudo-elements may only be appended after the last simple selector of the selector. See http://www.w3.org/TR/CSS2/selector.html#pseudo-elements (displayed in new window). |
| E756 | The pseudo-element/pseudo-class used was not recognised | Each CSS standard allows different pseudo-elements and pseudo-classes to be used, and one used here does not match the list of allowed pseudo-classes/pseudo-elements for this CSS Level. |
| E757 | A media type is missing | A media type was expected but appears to be missing. This can be caused by an extra comma in a list of media types. |
| E758 | The media type MEDIA_TYPE was not recognised | Each CSS standard allows different media types to be used, and the type used here does not match the list of allowed types for this CSS Level. |
| E759 | A media query contains a parsing error | The media query contains a colon (:) in the wrong place. |
| E760 | Unknown media feature: MEDIA_FEATURE | The media feature name is not recognised as valid for the CSS Level used for the validation. |
| E761 | TEXT_DESCRIBING_PROBLEM: RULES_FOR_THIS_FEATURE | The value used is invalid for this media feature, because the value does not follow the allowed rules for this feature. A list of valid rules will be displayed for the feature. |
| E762 | The 'to' and 'from' keywords must appear only once | The 'to' and 'from' keywords must appear only once in a keyframe selector. See http://www.w3.org/TR/css3-animations/#keyframes (displayed in new window). |
| E763 | Invalid percentage found in keyframe selector | Percentages must contain the '%' character and the value must be between 0% and 100%. See http://www.w3.org/TR/css3-animations/#keyframes (displayed in new window). |
| E776 | Unknown property: PROPERTY_NAME | The property name is not recognised as a valid name for the CSS Level used for the validation. |
| E777 | TEXT_DESCRIBING_PROBLEM: RULES_FOR_THIS_PROPERTY | The value used is invalid for this property, because the value does not follow the allowed rules for this property. A list of valid rules will be displayed for the property using the format used in the chosen standard. |
| E778 | TEXT_DESCRIBING_PROBLEM: RULES_FOR_THIS_DESCRIPTOR | The value used is invalid for this descriptor, because the value does not follow the allowed rules for this descriptor. A list of valid rules will be displayed for the descriptor using the format used in the chosen standard. |
| E779 | The '!important' keyword is not allowed here | The '!important' keyword is only allowed in property values and not in declaration values. |
| W776 | A vendor extension has been used | Authors should avoid using vendor extensions where possible. See http://www.w3.org/TR/CSS2/syndata.html#vendor-keywords (displayed in new window). |
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