rfc3986
2.1. Percent-Encoding > "%20" is the percent-encoding for the binary octet "00100000" (ABNF: %x20), which in US-ASCII corresponds to the space character (SP).rfc1866
8.2.1. The form-urlencoded Media Type 1. The form field names and values are escaped: space characters are replaced by `+', and then reserved characterscf. IPA URI のエスケープ