Comments in HTML

Why do the specs say that leading spaces in a comment block are illegal?

<! -- Why is this comment illegal? -- >

-- 
Russell O'Connor                           roconnor@uwaterloo.ca
    <URL:http://www.undergrad.math.uwaterloo.ca/%7Eroconnor/>
"And truth irreversibly destroys the meaning of its own message"
-- Anindita Dutta, "The Paradox of Truth, the Truth of Entropy"