TypeError when trying to edit Codex page prevents editing

[poena]

I'm trying to update the following page:
​https://codex.wordpress.org/Theme_Markup

I am logged in both to WordPress.org and to Codex.
When I click edit, (with or without selecting a section)
I am redirected to a page with nothing more than the message:

Exception encountered, of type "TypeError"

(My intention is to replace the HTML5 theme support example)

[dd32]

Please find a new home for the content, and I'll get the codex module fixed / page redirected

[poena]

Let's use ​https://developer.wordpress.org/themes/classic-themes/basics/theme-functions/

(It has examples and links to the developer reference for the add_theme_support function).