It's been deprecated for a while. I attempted to remove it last week, which led to T193191. Let's continue here to form a proper plan.
The root cause for T193191 was use of document.write(). In general, document.write() is only safe to use for the purpose of adding arbitrary HTML tokens into the HTML stream that the browser parses from the server, and only from an inline or synchronous script referenced in the HTML source.
Using it from an asynchronous script has never worked reliably in any browser. Trying to write from an async script, always behaves the same: It cannot add to the page because the page is already "closed" (e.g. reached </html>). Ideally, a browser would ignore this impossible instruction.
Instead, for compatibility with web sites that predate the existence of Wikipedia, browsers must assume the script intended to create a new document but forgot to call document.open() before document.write(), where document.open() will create a new page (replacing the current one). This causes the infamous blank page. To emphasise, this is not something MediaWiki does. Browsers do this intentionally for compatibility with very old web sites that are even older than our legacy scripts from 2010.
Untill 2013, MediaWiki loaded scripts synchronously, which meant document.write() just worked. After that, we've started to load scripts asynchronously. The good thing is, our users don't use document.write() for adding visible page content in a specific place. Instead, they typically used it to add invisible elements, such as <style>, <script>, or model dialogs. This meant that in 2015, after numerous reports about old scripts not working on various wikis, I decided to do something the browser cannot do: Magically make document.write() work, by removing the browser method, and replacing it with our own. Our version would invisibly parse the HTML and add it to the end of the page. (T108139, d861c6593a).
This has various maintainability issues and cannot be kept forever. For example, our "magic way" to make document.write work for our old scripts, has the down-side of also breaking other ways to use the method, for example, it caused site notices to sometimes appear at the bottom of the page (depending on how fast the page loaded) - see T125323 for details. This was caused by us, and would not have happened had we kept document.write unmagicified. Basically: Either we break modern content, or we break old scripts. There is no way to have both.
Our version of document.write has been deprecated with console warnings for three years, since 2015. There are still lots of search results (pasted at T193191#4163158), but the good news is that most are not active issues. Many are unused scripts (like unused templates), or technically loaded/used but do nothing or break due to unrelated issues before reaching the call to document-write. The number of actual visible issues that cause blank pages, should be small.
Unfortunately, as we saw today, this mechanism does not work the way I thought it would. While all browsers support it (and have for many years), it only affects top-level script execution. The trick does not work for indirect execution via timeouts, events, callbacks etc. And we load scripts in MediaWiki indirectly (usually via "Promise", or "requestIdleCallback", or "requestAnimationFrame"). After some research, I found browsers behave this way intentionally for compatibility reasons (https://www.w3.org/Bugs/Public/show_bug.cgi?id=9767) and to keep predictable behaviour for code that runs indirectly, impacting only top-level execution (sync vs async). I have an isolated test case (https://codepen.io/Krinkle/pen/pVjORL/?editors=0010), but the short version is: upstream browsers cannot help us.
- 2010 (MediaWiki 1.17):
- Script were synchronous, slow, and HTML display was interruptible by scripts.
- No ResourceLoader.
- And at this time, most browsers (except IE) allowed document.write() always, possibly blanking the page in certain cases.
- 2011 (MediaWiki 1.18):
- ResourceLoader introduced. Scripts were still synchronous and behave the same way as before.
- Wikibits deprecated.
- Meanwhile, Firefox and Safari started to ignore document.write() on async scripts. Other browsers followed.
- 2013 (MediaWiki 1.23):
- Wikibits deprecation warnings added to the browser console.
For the short-term we have no choice but to revert and keep our magical version of document.write(), because the version that browsers provide is not compatible with our legacy scripts. To avoid this problem in the future I propose a deadline to permanently remove this feature. After that, we will disable document.write() within MediaWiki.
- Update our document.write() to do "nothing" (except console warning, like now). This will match the behaviour of browsers for document.write in async scripts.
During this "phase 2", anything that relies on document.write() will fail but in an isolated way (That means, no blank pages, the page will work but without the script). These can then be found and addressed. The start of Phase 2 will also be in Tech News with the same links for support. This time reaching to look for local functionality that is missing or different.
- Scan for remaining uses and remove them, and remove our document.write().