Doesn't the browser console already output any errors related to parsing LESS files?

Does it?

Maybe it's specific to my development environment, but when I made an "accidental" error in one of the less files I saw this in the console:

I applied these two mistakes locally in my MediaWiki checkout, to observe the behaviour as of today:

--- a/resources/src/jquery/jquery.makeCollapsible.less
+++ b/resources/src/jquery/jquery.makeCollapsible.less
@@ -8,6 +8,7 @@
 
 /* See also jquery.makeCollapsible.js */
 .mw-collapsible-toggle {
+   .bla();
    float: right;
    -moz-user-select: none;
    -webkit-user-select: none;
diff --git a/resources/src/mediawiki.special/comparepages.less b/resources/src/mediawiki.special/comparepages.less
index ec10192d29..917683face 100644
--- a/resources/src/mediawiki.special/comparepages.less
+++ b/resources/src/mediawiki.special/comparepages.less
@@ -3,6 +3,7 @@
  */
 
 .mw-special-ComparePages {
+   .bla();
    .mw-htmlform-ooui-wrapper {
        width: 100%;
    }

The first one will load on any wiki page where e.g. <div class="mw-collapsible"><div> exists, which you could edit and save on your Main_Page to try. This is loaded through the jquery.makeCollapsible general JavaScript+CSS module.

The second one will load on Special:ComparePages via the mediawiki.special styles-only module.

Either of these will cause an uncaught Less_Exception_Compiler on the server-side in the load.php response. This uncaught exception, along with a detail stack trace, is printed by by default to the terminal tab where you run composer serve (if using https://www.mediawiki.org/wiki/Quickstart). It is also written to the mw-error.log file, which is enabled by default in WMF CI (printed at the end of relevant Jenkins jobs), and also enabled by default in Quickstart, MediaWiki-Docker, and any other dev environment where you either include DevelopmentSettings.php in LocalSettings.php, or otherwise enable $wgDebugLogFile or $wgDebugLogGroups['exception'], as per https://www.mediawiki.org/wiki/Manual:How_to_debug.

[87044] [Fri Mar 29 18:18:26 2024] [resourceloader] Generating module package failed: [Exception Less_Exception_Compiler( /Users/krinkle/Development/mediawiki/vendor/wikimedia/less.php/lib/Less/Tree/Mixin/Call.php:137) .bla is undefined in /Users/krinkle/Development/mediawiki/resources/src/jquery/jquery.makeCollapsible.less]
#0 /Users/krinkle/Development/mediawiki/vendor/wikimedia/less.php/lib/Less/Tree/Ruleset.php(149): Less_Tree_Mixin_Call->compile(Less_Environment)
#1 /Users/krinkle/Development/mediawiki/vendor/wikimedia/less.php/lib/Less/Tree/Ruleset.php(91): Less_Tree_Ruleset->EvalMixinCalls(Less_Tree_Ruleset, Less_Environment, integer)
#2 /Users/krinkle/Development/mediawiki/vendor/wikimedia/less.php/lib/Less/Tree/Ruleset.php(96): Less_Tree_Ruleset->compile(Less_Environment)
#3 /Users/krinkle/Development/mediawiki/vendor/wikimedia/less.php/lib/Less/Parser.php(188): Less_Tree_Ruleset->compile(Less_Environment)
#4 /Users/krinkle/Development/mediawiki/includes/ResourceLoader/FileModule.php(1127): Less_Parser->getCss()
#5 /Users/krinkle/Development/mediawiki/includes/ResourceLoader/FileModule.php(1019): MediaWiki\ResourceLoader\FileModule->compileLessString(string, string, MediaWiki\ResourceLoader\Context)

It is also reachable in the browser, in a comment prepended to the top of any load.php response. Both for CSS-only repsonses, and for combined JS/CSS responses, like so:

Lastly, for cases where the Less stylesheet is part of a combined JS+CSS module, or otherwise when the stylesheet is lazy-loaded, it is also printed to the browser's console.error channel:

I'd love for CSS-only errors to also be printed to the browser console, but I'm not sure there's a way to do that from an HTTP response with a text/css content type. If there is, let me know! In general though, I recommend developing with Quickstart or otherwise with mw-error.log tailed, so that you're always but one glance away from seeing any uncaught errors and other helpful warnings.

Change #1015614 had a related patch set uploaded (by Krinkle; author: Krinkle):

[mediawiki/core@master] ResourceLoader: Remove url/reqId noise from CSS/JS exception comments

https://gerrit.wikimedia.org/r/1015614

Change #1015614 merged by jenkins-bot:

[mediawiki/core@master] ResourceLoader: Remove url/reqId noise from CSS/JS exception comments

https://gerrit.wikimedia.org/r/1015614

In T116129#9674322, @Krinkle wrote:

I'd love for CSS-only errors to also be printed to the browser console, but I'm not sure there's a way to do that from an HTTP response with a text/css content type. If there is, let me know!

The content type could be arbitrary (by the time the server sets it, it knows there was an error) but I doubt something loaded via a <link> or <style> tag would ever be executed as JS code. A script in the main document could look through the CSS tags and find ones with errors, though. I don't think Javascript can access the content of URLs in <link> tags, but it has access to the CSS syntax tree so the error could be encoded there (e.g. #nosuchthing { content: "<stack trace>"; }). Probably too hacky to be worth it though.