From c560ab56c439cea2b8a9b82377a059b6d11404fe Mon Sep 17 00:00:00 2001
From: Anne van Kesteren <annevk@annevk.nl>
Date: Tue, 9 Sep 2025 18:31:21 +0200
Subject: [PATCH] Encourage web browser User-Agent value to start with
 `Mozilla/5.0 (`

This matches what all major web browsers already do and navigator.appVersion relies on this to a fault. See https://github.com/whatwg/html/issues/11630 for context.
---
 fetch.bs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fetch.bs b/fetch.bs
index c32c36a19..f00f3c966 100755
--- a/fetch.bs
+++ b/fetch.bs
@@ -1416,6 +1416,10 @@ downloads. This format of range header value can be set using <a>add a range hea
 <a>implementation-defined</a> <a for=/>header value</a> for the `<code>User-Agent</code>`
 <a for=/>header</a>.
 
+<p class=note>For unfortunate web compatibility reasons, web browsers are strongly encouraged to
+have this value start with `<code>Mozilla/5.0 (</code>` and be generally modeled after other web
+browsers.
+
 <p>The <dfn>document `<code>Accept</code>` header value</dfn> is
 `<code>text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8</code>`.