From https://once.com/writebook

test/models/page_test.rb

@@ -0,0 +1,10 @@
+require "test_helper"
+
+class PageTest < ActiveSupport::TestCase
+  test "html preview" do
+    page = Page.new(body: "# Hello\n\nWorld!")
+
+    assert_match /<h1>Hello<\/h1>/, page.html_preview
+    assert_match /<p>World!<\/p>/, page.html_preview
+  end
+end