Why?
The style sheets contain syntax errors. Browsers may not be able to render the page as expected.
How?
Check the style sheets with the W3C CSS Validator to find and correct CSS syntax errors.
Support for CSS varies from one device to the other. It is relatively safe to assume that mobile devices that support CSS support the CSS Level 1 grammar along with the @media attribute.
The CSS Validator checks style sheets against CSS Level 2.1 by default. CSS Level 1 may be selected prior to running the CSS validator in its "More options" menu. Unknown selectors, combinators, properties and values for the CSS Level 1 profile are reported as a separate warning by the mobileOK Checker (they are reported as errors by the CSS validator) and do not account for this message.
Where?
Triggered by https://elearning.sut.ac.th/theme/yui_combo.php?rollup/3.15.0/yui-moodlesimple-min.css.
More information
Severity: severe
Most mobile devices should render the page but the user experience is strongly impacted.
Category: Rely on Web standards
In the highly fragmented market of devices and browsers, standards are the best guarantee for interoperability.
Best practice:
Send content in a format that is known to be supported by the device.