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This page lists all standardized CSSOM constants in CSS to avoid the creation of conflicting specs. Vendor-specific features and experimental implementations must use numbers greater than 1000: the 0-1000 range is reserved by the CSS Working Group.
| 0 | - | Not assigned; was UNKNOWN_RULE in DOM-Level-2-Style |
| 1 | STYLE_RULE | CSSOM |
| 2 | - | Not assigned; was CHARSET_RULE in DOM-Level-2-Style |
| 3 | IMPORT_RULE | CSSOM |
| 4 | MEDIA_RULE | CSSOM |
| 5 | FONT_FACE_RULE | CSSOM |
| 6 | PAGE_RULE | CSSOM |
| 7 | KEYFRAMES_RULE | css3-animations |
| 8 | KEYFRAME_RULE | css3-animations |
| 9 | - | likely COLOR_PROFILE, SVG/CSS4 |
| 10 | NAMESPACE_RULE | CSSOM |
| 11 | COUNTER_STYLE_RULE | css3-lists |
| 12 | SUPPORTS_RULE | css3-conditional |
| 13 | DOCUMENT_RULE | css3-conditional |
| 14 | FONT_FEATURE_VALUES_RULE | css3-fonts |
| 15 | VIEWPORT_RULE | css-device-adapt |
Vendors can list their proprietary constants below to avoid cross-vendor overlap.