The CSS keyword 'default' is not allowed within CSS custom identifiers, which are used for many types of user-defined names in CSS (for example, names created by @keyframes rules, counters, @container names, custom layout or paint names). This adds 'default' to the list of names that are reserved from being used in custom identifiers, which already reserve 'inherit', 'initial', 'unset', 'revert', and 'revert-layer'.
Keywords that CSS uses (or is likely to use in the future) as values accepted by any CSS property should not be allowed in custom identifiers because many custom identifiers are also values of CSS properties. If they can be custom identifiers, then developers could create content that would be broken by the addition of these keywords as property values either to all CSS properties, or to a particular CSS property that already accepts custom identifiers.