The code list at https://boards.guro.cx/static/html/bbcode.html lists six formatting tag pairs:

​[​h​]​{text}​[​/h​]​
{text}
Formats {text} as headline.

​[​b​]​{text}​[​/b​]​
{text}
Makes {text} bold.

​[​i​]​{text}​[​/i​]​
{text}
Makes {text} italic.

​[​u​]​{text}​[​/u​]​
{text}
Underlines {text}.

​[​s​]​{text}​[​/s​]​
{text}
Creates a strikethrough on {text}.

​[​spoiler​]​{text}​[​/spoiler​]​
{text}
Masks {text} until it is marked or the mousepointer hovers over it.


There are some additional tags available:

​[​code​]​{text}​[​/code​]​

{text}

Renders {text} as code snippet.
​
​> (a closing angle bracket) at the beginning of a line renders this line
> with a quote color,
​< (a opening angle bracket) at the beginning of a line renders this line
< with another quote color.

​>​> (two closing angle brackets) directly followed (i.e. no blanks between) by the number of an existing post gives a link to that post, like >>5708. Unvalid links are striked through.
Since the posts are globally numbered it should be theoretically possible to link into other threads and boards, but it seems that this works only partly at this moment. There are links with a “Cross-thread”-flag that are working, although there is a pop-up message “Quote source not found” when hovering the mouse pointer over them. “Cross-board”-links seem not to be available.

Anticonfusion hint: The characters of the describing samples are preceded by, interrupted by or enclosed in "Zero Width Spaces" to prevent the conversion.
Because of their nature you can't see these characters, but you can detect them indirect: It takes more than one stroke of the cursor left or right keys to move the cursor from one viewable character to the next.
You can test this here: 123456 (no Zero Width Spaces, six keystrokes), 12​45​​89 (one Zero Width Space between 2 and 4, two Zero Width Spaces between 5 and 7, nine keystrokes)
To copy Zero Width Spaces it is helpful to include the leading and following characters and delete these after inserting.


You can insert special characters like arrows [← (U+2190), → (U+2192)], horizontal ellipses [… (U+2026)], dashes [– (U+2013), — (U+2014)] or even Zero Width Spaces [​ (U+200b)] ‒ to be precise all characters ‒ by typing their Unicode value (U+….) as Alt-codes in Windows or as Ctrl+Shift+U-sequence in Linux. It would be to escalating to explain the methods in detail. Do a search in the internet and you’ll find more than enough tutorials. You’ll learn all about which steps are to take and which limitations apply. Just one note: The input values have to be hexadecimal. It might be necessary to force Windows to accept hex-Alt-codes by activating an option in the registry ‒ look up its name “EnableHexNumpad” in the internet.
You also can copy and paste. Get the characters you want from an old post or ‒ like other characters ‒ from the character table program on your computer or smartphone.
Hint: It is always a good idea to check if the operation worked as intended. Sometimes characters are altered while moved to or from the clipboard. And sometimes different characters just look like they are the same, because they are presented similar by a program.


You are invited to provide information about now working, reenabled or (for the archive) "historical" formatting codes to be added.