Fix broken internal links when served from subdirectory

Internal links in Markdown sources use root-relative paths like
/ideas/print-backgrounds/, which break when the site is served from
the /csswg-wiki/ subdirectory on GitHub Pages.

Add a Jekyll plugin hook that prepends site.baseurl to absolute
internal href and src attributes in the rendered HTML. When baseurl
is empty (e.g. after switching to the wiki.csswg.org custom domain),
the hook is a no-op.

Author: sideshowbarker

_plugins/baseurl_links.rb

@@ -0,0 +1,20 @@
+# Prepend site.baseurl to absolute internal links in rendered HTML.
+#
+# Links in Markdown sources use root-relative paths like /ideas/foo/.
+# When the site is served from a subdirectory (baseurl: /csswg-wiki),
+# these need to become /csswg-wiki/ideas/foo/ in the final HTML.
+# When baseurl is empty (custom domain), this is a no-op.
+
+Jekyll::Hooks.register [:pages, :documents], :post_render do |item|
+  next if item.output_ext != ".html"
+
+  baseurl = item.site.config["baseurl"].to_s
+  next if baseurl.empty?
+
+  prefix = Regexp.escape(baseurl.delete_prefix("/"))
+
+  item.output = item.output.gsub(
+    /((?:href|src)=")\/(?!#{prefix}\/|\/)/,
+    "\\1#{baseurl}/"
+  )
+end