WEBGL_EXT is the appropriate answer for all such cases.
WEBGL_<VENDOR PREFIX> is for single vendor extensions.
WEBGL_WEBGL is reserved for Khronos ratified extensions that are specific to WEBGL.
WEBGL_WEBGL does look a bit odd. The equivalents in OpenGL & OpenGL ES are GL_ARB and GL_OES so they don't at first glance look so odd but they have the same meaning <API>_<prefix for WG Approved extensions>.