For a static page, HTML is good.

For a dynamic page in a Ruby application, you end up with Ruby + HTML + ERB + (in rails) partials with fuzzy unclear interfaces.

With Phlex you end up with Ruby + HTML, and objects have clear interfaces. Also performance is significantly better, and you can do some really neat things (example) with object composition. And Ruby is fun to write (for rubyists).

It’s not for everyone, but it’s a refreshing approach for ruby apps (not just rails!) that I really dig as a rubyist.

Plain HTML is not comparable to Phlex because plain HTML does not support dynamic values or embedding templates into other templates.

By “regular HTML”, do you mean what Ruby on Rails supports by default? That would be HTML embedded in a variant of ERB using Action View.

For comparison, here’s an (untested) example of HTML and ERB using Action View that demonstrates passing a score value down to a sub-template:

@score = calculate_score
# the view is implicitly called

<p>You scored <%= @score %> points.</p>
<%= render partial: "judgement", locals: {score: @score} %>

<div class="judgement">
  <% if score > 5 %>
    Great job!
  <% else %>
    You could do better.
  <% end %>
</div>

Based on Phlex’s documentation, I think it would let you write that Rails view like this:

render TestResults.new(
  score: calculate_score
)

class TestResults < Phlex::HTML
  def initialize(score:)
    @score = score
  end

  def template
    p { "You scored #{@score} points." }
    judgement
  end

  def judgement
    div(class: 'judgement') do
      if @score > 5
        'Great job!'
      else
        'You could do better.'
      end
    end
  end
end