diff --git a/README.rst b/README.rst
index 168af93..cba47d9 100644
--- a/README.rst
+++ b/README.rst
@@ -3,4 +3,31 @@ urlparse
 
 urlparse is a URL parser compatible to `url_parser_parse_url()` from
 `nodejs/http-parser <https://github.com/nodejs/http-parser>`_ which
-has been archived since 2022.
+has been archived since 2022.  urlparse only implements the strict
+mode.
+
+There is a slight difference in a return code when they fail.
+`url_parser_parse_url()` returns nonzero if it fails.
+`urlparse_parse_url()` returns the negative error code
+``URLPARSE_ERR_PARSE`` if it fails.
+
+`url_parser_parse_url()` historically does not follow any standards
+like RFC 3986.  Here is the allowed characters in each URL component:
+
+- scheme: ``A-Za-z``
+- userinfo: ``A-Za-z!$%&'()*+,-.:;=_~``
+- host: ``a-zA-Z0-9-.``
+- IPv6 host: ``A-Fa-f0-9.:``
+
+  - optionally followed by zone info which starts ``%`` and can
+    contain: ``A-Za-z0-9%-._~``
+
+  - and IPv6 host must be enclosed by ``[`` and ``]``
+
+- port: ``0-9``
+- path: ``A-Za-z0-9!"$%&'()*+,-./:;<=>@[\]^_`{|}~``
+- query: ``A-Za-z0-9!"$%&'()*+,-./:;<=>?@[\]^_`{|}~``
+- fragment: ``A-Za-z0-9!"#$%&'()*+,-./:;<=>?@[\\]^_\`{|}~``
+
+  - all consecutive ``#`` characters that precede a fragment are
+    treated as a single ``#``.  A fragment cannot start with ``#``.