The DTD and spec mention a <base> sub-element of <collations>/<collation> which has never been used in any CLDR release, and I believe it is redundant with <import>. Can we remove <base> from the DTD? If not, please deprecate it.
Note: I have had ideas of doing something like <base> that is different from <import>, and possibly more efficient, but have not yet figured out how to make it work reasonably. I would probably not permit arbitrary bases. (Only special ones like search/eor/unihan, and prebuild them.)
The optional base element <base>...</base>, contains an alias element that points to another data source that defines a base collation. If present, it indicates that the settings and rules in the collation are modifications applied on top of the respective elements in the base collation. That is, any successive settings, where present, override what is in the base as described in Setting Options. Any successive rules are concatenated to the end of the rules in the base. The results of multiple rules applying to the same characters is covered in Orderings.