diff --git a/index.d.ts b/index.d.ts
index 0dd9203..42948b7 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -285,6 +285,8 @@ export type Options = {
 
 URLs with custom protocols are not normalized and just passed through by default. Supported protocols are: `https`, `http`, `file`, and `data`.
 
+Human-friendly URLs with basic auth (for example, `user:password@sindresorhus.com`) are not handled because basic auth conflicts with custom protocols. [Basic auth URLs are also deprecated.](https://developer.mozilla.org/en-US/docs/Web/HTTP/Authentication#access_using_credentials_in_the_url)
+
 @param url - URL to normalize, including [data URL](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs).
 
 @example
diff --git a/readme.md b/readme.md
index 02ae2ba..45cf82c 100644
--- a/readme.md
+++ b/readme.md
@@ -32,6 +32,8 @@ normalizeUrl('//www.sindresorhus.com:80/../baz?b=bar&a=foo');
 
 URLs with custom protocols are not normalized and just passed through by default. Supported protocols are: `https`, `http`, `file`, and `data`.
 
+Human-friendly URLs with basic auth (for example, `user:password@sindresorhus.com`) are not handled because basic auth conflicts with custom protocols. [Basic auth URLs are also deprecated.](https://developer.mozilla.org/en-US/docs/Web/HTTP/Authentication#access_using_credentials_in_the_url)
+
 #### url
 
 Type: `string`