diff --git a/userstyles/github-fixed-width-textarea.user.css b/userstyles/github-fixed-width-textarea.user.css
new file mode 100644
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+++ b/userstyles/github-fixed-width-textarea.user.css
@@ -0,0 +1,20 @@
+/* ==UserStyle==
+@name          GitHub Fixed-Width Textarea
+@description   Enforce fixed-width font and font-size in GitHub textareas
+@version       1.0.0
+@license       CC0-1.0
+@author        Jim Myhrberg (https://jimeh.me/)
+@namespace     jimeh.me
+@homepageURL   https://github.com/jimeh/dotfiles/blob/main/userstyles/github-fixed-width-textarea.user.css
+@updateURL     https://github.com/jimeh/dotfiles/raw/main/userstyles/github-fixed-width-textarea.user.css
+@preprocessor  stylus
+
+@var text ghfwt-font-size "Font Size" 12px
+==/UserStyle== */
+@-moz-document domain("github.com") {
+  textarea {
+    font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, Courier,
+      monospace !important;
+    font-size: ghfwt-font-size !important;
+  }
+}