Wikipedia:Manual of Style/Hidden text

On Wikipedia, hidden text is text that is visible when editing the source for the page or when using VisualEditor, but not on the "rendered" version of the page presented to readers of the article.

Invisible comments are useful for alerting other editors to issues such as common mistakes that regularly occur in the article, a section title's being the target of an incoming link, or pointing to a discussion that established a consensus relating to the article. They should not be used to instruct other editors not to perform certain edits, although where existing local consensus is against making such an edit, they may usefully draw the editor's attention to that. Avoid adding too many invisible comments because they can clutter the wiki source for other editors.

Appropriate uses for hidden text

 * If you change this section title, also change the links to it on the pages .... (bots do this, see MOS:RENAMESECTION)
 * When adding table entries, remember to update the total given in the text.
 * Please only add items to this section if they are mentioned in, and cited to, reliable sources.

Some situations where hidden text is useful and good to have in articles:
 * Instructing others how to edit a page that may be difficult to edit
 * Providing information to assist other editors in preventing a common mistake. For example, if there is a reference which is known to be wrong, it may be appropriate to let other editors know about the error to prevent a likely re-insertion of the error.
 * Warning editors against making edits that would breach a specific discretionary sanction that has been placed on the page.
 * Informing other editors that a section title is linked from a redirect, so that if the title is altered, the redirect can be changed.
 * In this case, it is often better to use an anchor in the first place.
 * Letting others know of a better article or location within the article to add information on a current event or other hot topic where there is temptation to add it in that location.
 * Letting others know the location of a template that is included on a page, most commonly an embedded list.
 * Reminding others of Wikipedia policies where they have been frequently broken. For example, in many articles, hidden text is necessary to remind editors not to add inappropriate links. On the page Help:Getting started, hidden text is used to let others know not to write their first article on that page.
 * Preparing small amounts of information to be added to the article in the future (such as when a known event will occur). Larger amounts of information should be prepared on a subpage of the article's talk page or in user space.
 * Hiding a portion of the text that has been temporarily removed while consensus is pending. However, it may be preferable to transfer such text onto a subpage of the article's talk page.
 * Commenting out categories for articles in user pages and user subpages. See: Categorizing user pages
 * Cues for bots, such as the  on categories being discussed at Categories for discussion
 * Commenting on maintenance tags such as citation needed and disambiguation needed, to detail to other editors how you have tried to solve a problem but failed; to save them the effort of merely reproducing your failure.

Inappropriate uses for hidden text
Types of hidden text that should not be added include:
 * This article is perfect. Don't mess with it!
 * To the bozos who keep editing this section to use em-dashes: eat my shorts.
 * BTW this album slaps hard, check it out if you get a chance
 * Jimmy was here.
 * Asserting ownership of an article
 * Telling others not to edit an article, period
 * Telling others not to perform certain edits to a page, unless there is an existing guideline or policy against that edit.
 * When it is only a local consensus that a certain edit should not be performed, the hidden text should be worded more softly to suggest to the editor to consult the talk page (or archive page if appropriate) for the current consensus prior to making the edit. Since consensus can change, it is inappropriate to use hidden text to try to prohibit making a certain edit merely because it would conflict with an existing local consensus.
 * Disruptive behavior, such as vandalism, harassment, threats, or libel content
 * Using it as a talk page
 * Advertising WikiProjects.

How to enter hidden text
Enter This will render as: The clock tower was constructed in 1928,[1] and while it was struck by lightning in November 1955,[2] it remained functional until 1987,

when its face was permanently damaged in a fight between a high school student and an immortal vampire.[3]

Check that your invisible comment does not upset the article's formatting, for example by introducing unwanted white space.