Even though vendor-specific @-rules are guaranteed not to cause conflicts, it should be recognized that these extensions may also be subject to change at the vendor’s whim, as they don’t form part of the CSS specifications, even though they often mimic the proposed behavior of existing or forthcoming CSS @-rules. Thus, it is not recommended to use them in production code.

The rule raises an issue when one of the following prefixes is found:

-ah- Antenna House
-apple- Webkit supports properties using the -apple- prefixes as well
-atsc- Advanced Television Standards Committee
-epub- EPUB
-fx- JavaFX
-hp- Hewlett Packard
-khtml- Konqueror browser
-moz- Mozilla Foundation (Gecko-based browsers)
-ms- Microsoft
mso- Microsoft Office
-o- Opera Software
prince- YesLogic
-rim- Research In Motion
-ro- Real Objects
-tc- Tall Components
-wap- The WAP Forum
-webkit- Safari, Chrome (and other WebKit-based browsers)
-xv- Opera Software

This rule also raises an issue when one of the following experimental @-rules is found:

`annotation` #2	`bottom-center`	`bottom-left`
`bottom-left-corner`	`bottom-right`	`bottom-right-corner`
`character-variant` #2	`counter-style`	`custom-media`
`document` #2	`font-feature-values` #2	`keyframes`
`left-bottom`	`left-middle`	`left-top`
`ornaments` #2	`right-bottom`	`right-middle`
`right-top`	`styleset` #2	`stylistic` #2
`swash` #2	`top-center`	`top-left`
`top-left-corner`	`top-right`	`top-right-corner`
`viewport`

Noncompliant Code Example

.mybox {
  @-moz-vendoratrule {} /* Noncompliant: vendor-prefixed @-rule */
  @custom-media --narrow-window (max-width: 30em); /* Noncompliant: experimental @-rule */
}