Some implementations need much finer-grained support for editing. For example, Google Docs implements a complete text editing framework.
This is extremely difficult in JS currently, yet every modern OS has support for the low-level APIs that make it efficient and practical. These include the ability to find the width of a string, but also the ability to find out where cursor positions would be, etc. Should include support for BIDI positioning also.
Text editing support is beyond the scope of TC 39, and should be proposed to a W3C working group instead. See discussion at