Standards developed by the WHATWG are licensed under CC0 (variant of Public Domain that works globally). The HTML Standard is licensed under MIT for historical reasons. This is important for these reasons:
Standards should be reusable everywhere without there being a legal grey area. The IDL code will end up in software, examples and text will end up in articles and books, either literally or modified as required. This and all that which we cannot foresee must be possible without copyright getting in the way.
If the person or entity maintaining the standard takes it in the wrong direction or ceased maintenance it is paramount for the community depending on the standard to work on an alternative without copyright getting in their way.
The W3 Project at CERN had the same policy as the WHATWG: The definition of protocols such as HTTP and data formats such as HTML are in the public domain and may be freely used by anyone. Tim BL Nowadays the W3C employes a more restrictive W3C document license that does not allow for derivative works and requires attribution.