DEV: Add apply_modifier in Email::Renderer for html modifications (#25205)

lib/email/renderer.rb

@@ -32,6 +32,7 @@ module Email
 
       style.format_basic
       style.format_html
+      DiscoursePluginRegistry.apply_modifier(:email_renderer_html, style, @message)
       style.to_html
     end
   end

spec/lib/email/renderer_spec.rb

@@ -11,14 +11,28 @@ RSpec.describe Email::Renderer do
     mail.html_part =
       Mail::Part.new do
         content_type "text/html; charset=UTF-8"
-        body "<h1>Key &amp; Peele</h1>"
+        body "<h1>Key &amp; Peele</h1> <a href=\"https://discourse.org\">Discourse link</a>"
       end
 
     mail
   end
 
+  let(:renderer) { Email::Renderer.new(message) }
+
   it "escapes HTML entities from text" do
-    renderer = Email::Renderer.new(message)
     expect(renderer.text).to eq("Key & Peele")
   end
+
+  context "with email_renderer_html modifier" do
+    it "can modify the html" do
+      Plugin::Instance
+        .new
+        .register_modifier(:email_renderer_html) do |styles, _|
+          styles.fragment.css("a").each { |link| link["href"] = "httpz://hijacked.sorry" }
+        end
+
+      expect(renderer.html).not_to include("href=\"https://discourse.org\"")
+      expect(renderer.html).to include("href=\"httpz://hijacked.sorry\"")
+    end
+  end
 end