alt Hacker NewsAfter I read this, I started to look at the Wikipedia article on Base64 and eventually got to the article for the data URI scheme. That's where I found a sentence that seems to a little bit at odds with the blogpost. The Wikipedia article mentions that "whitespace characters are not permitted in data URIs".
But then I suppose it goes back to the main thrust of the blogpost because it says that in the context of HTML 4 and 5, that linefeeds within an attribute value are ignored. So possibly there are some other contexts where whitespace might not be ignored.
They are not, but you can encode them, if you encode whitespace characters, you included whitespace in a URL.
One of the requirement of URLs is that it needs to be transmissible over paper or aural media, so arbitrary octets and the unused portion of ASCII are not legal either.