Regarding "å" et al., from the HTML 3.0 specification:

   Character sets
          The charset parameter (as defined in section 7.1.1 of RFC 1521)
          may be used with the text/html content type to specify the
          encoding used to represent the HTML document as a sequence of
          bytes. Normally, text/* media types specify a default of
          US-ASCII for the charset parameter. However, for text/html, if
*         the byte stream contains data that is not in the 7-bit US-ASCII
*         set, the HTML interpreting agent should assume a default
*         charset of ISO-8859-1.

Especially note the last three lines. It is explicitly said that
ISO 8859-1 is the standard text charset for HTML documents.


Regarding "~" in URLs, from the HTTP 1.0 specification:

    3.2.1 General Syntax

   URIs in HTTP/1.0 can be represented in absolute form or relative to
   some known base URI [9], depending upon the context of their use. The
   two forms are differentiated by the fact that absolute URIs always
   begin with a scheme name followed by a colon.


       URI            = ( absoluteURI | relativeURI ) [ "#" fragment ]

       absoluteURI    = scheme ":" *( uchar | reserved )

       relativeURI    = net_path | abs_path | rel_path

       net_path       = "//" net_loc [ abs_path ]
       abs_path       = "/" rel_path
       rel_path       = [ path ] [ ";" params ] [ "?" query ]

       path           = fsegment *( "/" segment )
       fsegment       = 1*pchar
       segment        = *pchar

       params         = param *( ";" param )
       param          = *( pchar | "/" )

       scheme         = 1*( ALPHA | DIGIT | "+" | "-" | "." )
       net_loc        = *( pchar | ";" | "?" )
       query          = *( uchar | reserved )
       fragment       = *( uchar | reserved )

       pchar          = uchar | ":" | "@" | "&" | "="
       uchar          = unreserved | escape
       unreserved     = ALPHA | DIGIT | safe | extra | national

       escape         = "%" hex hex
       hex            = "A" | "B" | "C" | "D" | "E" | "F"
                      | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT

       reserved       = ";" | "/" | "?" | ":" | "@" | "&" | "="
       safe           = "$" | "-" | "_" | "." | "+"
       extra          = "!" | "*" | "'" | "(" | ")" | ","
*      national       = <any OCTET excluding CTLs, SP,
*                        ALPHA, DIGIT, reserved, safe, and extra>

*  For definitive information on URL syntax and semantics, see RFC 1738
*  [4] and RFC 1808 [9]. The BNF above includes national characters not
*  allowed in valid URLs as specified by RFC 1738, since HTTP servers are
*  not restricted in the set of unreserved characters allowed to
*  represent the rel_path part of addresses, and HTTP proxies may receive
*  requests for URIs not defined by RFC 1738.

Especially note the last lines. They clearly say that ISO 8859-1
characters like "Å", "Ä", "Ö" and "~" are allowed in URLs, since
they are part of "national" and not part of "reserved". It is
also explicitly said that this differs from the URL definition in
the RFCs.