diff --git a/src/html5ever/url.rs b/src/html5ever/url.rs
index b587efb98..2d5f60e07 100644
--- a/src/html5ever/url.rs
+++ b/src/html5ever/url.rs
@@ -335,17 +335,40 @@ pub unsafe extern "C" fn url_get_port(url: *const Url) -> i32 {
     }
 }
 
+fn clean_hostname_input(url: &Url, raw: &str) -> String {
+    let special = matches!(
+        url.scheme(),
+        "http" | "https" | "ws" | "wss" | "ftp" | "file"
+    );
+    let mut out = String::with_capacity(raw.len());
+    let mut in_brackets = false;
+    for c in raw.chars() {
+        match c {
+            '\t' | '\n' | '\r' => continue,
+            '[' => in_brackets = true,
+            ']' => in_brackets = false,
+            '/' | '?' | '#' => break,
+            '\\' if special => break,
+            ':' if !in_brackets => break,
+            _ => {}
+        }
+        out.push(c);
+    }
+    out
+}
+
 /// WHATWG `hostname` setter: sets the host without touching the port.
 #[no_mangle]
 pub unsafe extern "C" fn url_set_hostname(url: *mut Url, ptr: *const c_uchar, len: usize) -> i32 {
     let url = unsafe { &mut *url };
 
-    let slice = match str_from(ptr, len) {
+    let raw = match str_from(ptr, len) {
         Some(s) => s,
         None => return -1,
     };
+    let host = clean_hostname_input(url, raw);
 
-    match url.set_host(Some(slice)) {
+    match url.set_host(Some(host.as_str())) {
         Ok(()) => 0,
         Err(_) => -1,
     }
@@ -406,6 +429,18 @@ pub unsafe extern "C" fn url_set_host(url: *mut Url, ptr: *const c_uchar, len: u
         None => return -1,
     };
 
+    // The WHATWG host setter parses in "host state", which stops at the first
+    // '/', '?', '#' (and '\' for special schemes); the remainder is ignored
+    // rather than making the whole value invalid.
+    let special = matches!(
+        url.scheme(),
+        "http" | "https" | "ws" | "wss" | "ftp" | "file"
+    );
+    let end = input
+        .find(|c| c == '/' || c == '?' || c == '#' || (special && c == '\\'))
+        .unwrap_or(input.len());
+    let input = &input[..end];
+
     // Find the port separator ':', but only outside an IPv6 [...] literal.
     let colon = if input.starts_with('[') {
         input
@@ -430,7 +465,11 @@ pub unsafe extern "C" fn url_set_host(url: *mut Url, ptr: *const c_uchar, len: u
     // the WHATWG host setter ignores: set the host only, leaving any existing
     // port untouched (matches Chrome and Firefox).
     if port_str.is_empty() {
-        return if url.set_host(Some(host)).is_ok() { 0 } else { -1 };
+        return if url.set_host(Some(host)).is_ok() {
+            0
+        } else {
+            -1
+        };
     }
 
     // Validate the port up-front so we never apply the host and then fail.