Add a quick TIL on the limits of :visited styles

src/_til/2024/2024-12-25-limits-on-visited-styles.md

@@ -0,0 +1,40 @@
+---
+layout: til
+title: There are limits on the styles you can apply with `:visited`
+date: 2024-12-25 20:04:39 +0000
+tags:
+  - css
+summary: |
+  Because the `:visited` selector will tell you whether somebody has been to a URL, browsers limit what styles you can apply to such links -- to prevent somebody nefarious stealing your browsing history.
+---
+I was tweaking the `a:visited` style on this site, and I was confused about why this wasn't working:
+
+```
+a:visited {
+  text-decoration-style: dashed;
+}
+```
+
+I thought it might be a browser bug, or maybe some bad interaction between other `text-decoration` rules, but it turns out to be an intentional choice.
+
+A [Stack Overflow answer](https://stackoverflow.com/a/35037025/1558022) pointed to a page on MDN [Privacy and the :visited selector](https://developer.mozilla.org/en-US/docs/Web/CSS/Privacy_and_the_:visited_selector#limits_to_visited_link_styles), which explains:
+
+> Before about 2010, the CSS :visited selector allowed websites to uncover a user's browsing history and figure out what sites the user had visited. […] To mitigate this problem, browsers have limited the amount of information that can be obtained from visited links.
+
+and there are only a few styles you can apply to `:visited` links:
+
+> You can style visited links, but there are limits to which styles you can use. Only the following styles can be applied to visited links:
+>
+> * `color`
+> * `background-color`
+> * `border-color` (and its sub-properties)
+> * `column-rule-color`
+> * `outline-color`
+> * `text-decoration-color`
+> * `text-emphasis-color`
+> * The color parts of the `fill` and `stroke` attributes
+
+This is annoying for my purposes, but it makes sense, and I'm glad to know it's not my own incompetence that was preventing this CSS from working!
+
+I briefly considered trying to find ways around this, like doing something with variables or trying to trick the browser somehow, but I decided against it.
+It'd be a fragile hack, and any loophole I found should be reported and fixed rather than used for my own enjoyment.