URL-safe
URL-safe とは、URL として解釈可能であることで、URL-unsafe とは、そのままだとエンコードしなければ URL として解釈できないこと、として利用されているようだ。
出典はどこなのかと思ったが、URL の RFC に Unsafe に関する記載があるので、ここが参照されているのだろうか。 Unsafe:
Characters can be unsafe for a number of reasons. The space character is unsafe because significant spaces may disappear and insignificant spaces may be introduced when URLs are transcribed or typeset or subjected to the treatment of word-processing programs. The characters "<" and ">" are unsafe because they are used as the delimiters around URLs in free text; the quote mark (""") is used to delimit URLs in some systems.
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All unsafe characters must always be encoded within a URL. For example, the character "#" must be encoded within URLs even in systems that do not normally deal with fragment or anchor identifiers, so that if the URL is copied into another system that does use them, it will not be necessary to change the URL encoding.