How to build derived configurations?

[otherdaniel]

In the current drafts, there is no way to build a configuration based on another one. In order to allow e.g. a single additional custom element on top of the defaults, the developer would need to copy the default config from the spec, add the one lement, and paste that into their app. That seems clumsy.

(And also goes a little against the "ever-green" aspect, in that this might cause outdated defaults to be persisted.)

It might be better to have a way a convenient way to build one config on top of another; in particular on top of the defaults.

[otherdaniel]

Alternatives I can think of:

• Duplicate config properties with add/remove values. E.g., allowElements and addAllowElements.
• Specify a "parent", plus some semantics on how a config can be merged with the parent to get the effective config.
• Have some config operators, e.g. mergeConfig(config1, config2).
• Probably the simplest would be to make configs - at least the default config - accessible as a read-only object, so that devs can build their own derivatives. (E.g. a static Sanitizer.defaultConfigValue, or alternatively a .config attribute on every sanitizer instance.)