Something about the formatting of the code blocks used is all messed up for me. Seems to be independent on browser, happens in both Firefox and Chrome.
for (string line; getline(is,line); )
s.insert(line);
On my Firefox on Linux, this HTML page is not rendered with any custom typefaces, but it uses those specified by me as defaults for serif/sans serif/monospace.
The C++ code is rendered in my browser with my default, i.e. with JetBrains Mono and there is nothing weird.
The code quoted by you is indented as expected, not as in your posting.
On my computer, I have mostly typefaces that I have bought myself and which are seldom encountered in most computers. I do not have any of the typefaces that are typically specified in CSS rules, i.e. none of the typefaces that can be found in default installations of Windows, Linux or MacOS.
So perhaps there is a bug in their CSS at the definition of "wp-block-code", which on other computers selects a bad typeface that is proportional, so that the narrow spaces make the indentation disappear. (Their wp-block-code says "font-family:inherit" and I have not searched further to see from where the wrong font-family may be inherited.)
Here, perhaps because that bad typeface cannot be found, the browser uses my default monospace font and the code is displayed fine.
Or else, perhaps you have not set in your browser a proper default for monospace fonts and it just takes Arial or other such inappropriate system font even for monospace.
I have come to find this category of error to be distressingly large.
If a proportional font is used for rendering, the most likely cause is that the user has not configured the default monospace font in the settings of the browser.
The font is selected by the HTML/CSS of the ACM site, not by Bjarne.
There may be a bug in the CSS of the ACM site, but I think that it is more likely that anyone who does not see correctly formatted code on that page has forgotten to open the settings of their browsers and select appropriate default fonts for "serif", "sans serif" and "monospace".
As installed, most browsers very seldom have appropriate default fonts, you normally must choose them yourself.
In this case, whoever does not see a monospace font, which is mandatory for rendering the code on that page, because the indentation is done with spaces, which become too narrow if rendered with a proportional font, must have that proportional font set in their browser as a default monospace font, so they should correct this.
I didn't see the author at first. However, immediately after seeing the code I checked for the author, because I was sure it was Stroustrup.
Normal people who have a modern environment would std::println but Bjarne insists on using the I/O streams from last century instead
On my browser, all the code is properly indented, most likely because my browsers are configured correctly, i.e. with a monospace font set as the default for "monospace".
Whoever does not see indentation, most likely has not set the right default font in their browser.
My browser has an appropriate default monospace font (JetBrains Mono), so the code is formatted and indented correctly, as expected.
Where this does not happen, the setting for the default monospace font must be wrong, so it should be corrected.
Here the code is displayed with my default monospace font, as configured in browsers, so the formatting is fine.
There are only 2 possible reasons for the bad formatting: a bug in the CSS of the ACM site, which selects a bad font on certain computers or a bad configuration of your own browsers, where you have not selected appropriate default fonts.
Most of programming language conferences are organized by ACM.