https://mimesniff.spec.whatwg.org/#parsing-a-mime-type

[[
While sequence[s] is a whitespace character, continuously execute the following steps:

    Increment s by 1. 
]]

The loop does not check if `sequence[s]` exists. An invalid media type like `   ` either raises an out-of-bounds error or reads memory outside of `sequence`. All other loops include the check "If sequence[s] is undefined, return undefined.".