For me it was not just replacing ~E with ~H and changing the HTML attribute syntax. I got undefined function assigns/0 each time and had to add assigns = %{} each time even though the ~H snippet did not use any assign. See also "undefined function assigns/0" while trying to use Phoenix.LiveViewTest.rendered_to_string/1 · Issue #1798 · phoenixframework/phoenix_live_view · GitHub

In case this is useful for anyone, here is a working example of composing safe HTML text using an ~H block:

defmodule EmailBody do
  import Phoenix.Component, only: [sigil_H: 2]

  def compose(question_of_the_day) do
    assigns = %{
      question: question_of_the_day
    }

    ~H"""
    <div><%= @question %>?</div>
    """
    |> Phoenix.HTML.Safe.to_iodata()
    |> List.to_string()
  end
end

$ iex -S mix
...
iex(20)> "Why did the chicken cross the road (and here is a rogue div: <div>)" |> EmailBody.compose()
"<div>Why did the chicken cross the road (and here is a rogue div: &lt;div&gt;)?</div>"

The rogue <div> in the template parameter is safely escaped in the resulting string.