Constant SUBGROUP_FORM_FORBIDDEN_BITS 

const SUBGROUP_FORM_FORBIDDEN_BITS: u8 = 0xC0;

Bits 6 and 7, which the subgroup header’s 0b00X1XXXX form leaves clear.

Narrower than the equivalent on draft-19, whose form is 0b0XX1XXXX and which therefore admits 0x50..0x5F and 0x70..0x7F as well. Draft-16 Section 10.4.2 names only “the ranges 0x10..0x1F and 0x30..0x3F”.