Add HTML content to rendered web pages?

I’m probably misunderstanding your problem, but wouldn’t it be easier to write a module with functions containing the headings with links and use them both in the pages and in the ToC?

defmodule Headings do
  # not sure it would work

  titles_for_routes = %{
    Routes.page_path(YourWeb.Endpoint, :index) => "This is the Index PAGE",
    # ...
  }

  Enum.map(titles_for_routes, fn {route, title} ->
    def title(unquote(route)), do: unquote(title)
  end)

  def title(_unmatched), do: "Some default title"
end

# on the page
<title><%= Headings.title(@conn.private[:phoenix][:route]) # or whereever phoenix stores it's current route %></title>
# in the toc
<ul>
  <%= Enum.map(@routes, fn route -> %>
    <li><%= link(Headings.title(route), to: route) %></li>
  <% end) %>
</ul>

Thanks for the thoughtful and detailed reply. The problem with this approach, in my case, is that I have a number of places and ways in which headings are generated. For example:

• Some headings are hard-wired into the templates.
• Some headings are generated automagically, along with page sections, if the relevant data is available.
• Some headings are encoded in Markdown source code.

Because I’d rather not couple the TOC generation to each of these spots in the code, I’m thinking about a post-processing approach: if there’s a heading in the resulting HTML, I index it.

Here are some links that may help to show what’s going on:

• a rendered page for an item
• source data for the item page
• a rendered page for an article
• source data for the article