There are some variables that should just not be declared in hook_theme() or they run the risk of conflicting with attributes used by drupal_render() and/or the FAPI.
Used internally by drupal_render() - definitely don't use these for theme variable names:
access
printed
cache
type
defaults_loaded
pre_render
children
theme
render_children
markup
states
attached
theme_wrappers
post_render
prefix
suffix
Used internally by theme() - definitely don't use these in hook_theme() unless you really know what you're doing:
theme_hook_original
theme_hook_suggestion
theme_hook_suggestions
directory
attributes
title_attributes
content_attributes
Not covered above, but has special meaning within the core FAPI - generally discouraged to avoid human misunderstandings, strongly discouraged for theme hooks commonly used in/around forms unless you're implementing exactly what is documented at https://api.drupal.org/api/drupal/developer%21topics%21forms_api_referen...
after_build
ajax
array_parents
autocomplete_path
collapsed
collapsible
cols
default_tab
default_value
delta
description
disabled
element_validate
empty
empty_option
empty_value
field_prefix
field_suffix
group
header
js_select
languages
maxlength
multiple
options
parents
placeholder
process
required
resizable
return_value
rows
size
title
title_display
tree
value_callback
weight
This should all be clearly documented in the docblock for hook_theme as many themers would be unaware of all the items in the full list.
Postponed: needs info
11.0 🔥
documentation
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
The work here is about the Drupal 7 form api page and Drupal 7 is EOL. The work on the list stopped in 2013 around the time Drupal 8 was released. And the issue summary refers to a page with no content. It not clear any more what variables need documentation.
Is there work to be done here?