Why is this an issue?

Vendor-prefixed at-rules such as @-webkit-keyframes or @-ms-viewport are legacy browser-specific syntax. Modern browsers support the standardized unprefixed forms.

Keeping these prefixed at-rules in source code makes stylesheets harder to read and maintain. Replace the prefixed at-rule with its standardized form.

Noncompliant code example

@-webkit-keyframes { 0% { top: 0; } }

@-ms-viewport { orientation: landscape; }

Compliant solution

@keyframes { 0% { top: 0; } }

@viewport { orientation: landscape; }

Resources

Documentation