Dynamically changing template contents of a web form

Let's say I want to build a web app which would use templates in the form similar to the following:

<h1>
  { status_text[step] }
</h1>
<progress max="10" value="{ value[step] }">
</progress>

In other words, let's say that I have lists value[] and status_text[], change only the variable step over time and want the form to reflect these changes in the real-time (ideally, even without sending a notification from the code that changes step, so that I can use current_time instead of some artificial step). What technology should I use to implement such behaviour in the most elegant way? (for the main language, I'd prefer using Python but JS/PHP would also work if there's no Python-related solution)

Thanks in advance for any advice.