Mandatory extensions There is a common requirement for the design of a language on the web that it should allow for extensions, but it must allow a clear declaration as to whether understanding of an extension is a requirement to understanding of the document or whether it may be ignored. (See Evolvability) Historically the lack of such a "mandatory field" has led to a complete inabaility to get any particular guaranteed behaviour be clients on the web. This is essential for partial understan...
