Wikipedia:Linter

The Linter extension is a MediaWiki extension that aims to identify "lint": syntax errors in the code of Wikipedia pages. The lint in this case is broken and problematic markup on all wiki pages that cannot be fixed automatically by MediaWiki. The extension produces a list of these errors at Special:LintErrors, which editors and bots can consult to find pages that need attention. High-priority Linter issues require fixing as they may cause pages to display in undesirable fashion. The MediaWiki wiki help page describes 18 specific types of lint errors.

Background
A linter is software that helps an author or editor of a document (such as a wiki page or a programming file) see if there may be errors in the document. The extension does this for wiki pages: it helps identify whether a page displays as the author intended in some cases (for example, some image options are "linted" for), and helps identify whether a page displays as the author intended, due to changes in how the MediaWiki system creates HTML from wikitext. Further reasons can be found at mw:Help:Extension:Linter § Why and what to fix.

List of lint errors
From Special:LintErrors

High priority

 * Table tag that should be deleted
 * Misnested tag with different rendering in HTML5 and HTML4
 * Miscellaneous issues
 * Multiline table in list
 * Multiple unclosed formatting tags
 * Paragraph wrapping bug workaround
 * Self-closed tags
 * Old behaviour of link-wrapping font tags
 * Whitespace parsing bug
 * Unclosed quote in heading

Medium priority

 * Bogus file options
 * Fostered content
 * Misnested tags
 * Multi colon escape
 * Links in links

Low priority

 * Missing end tag
 * Missing end tag in heading
 * Obsolete HTML tags
 * Stripped tags
 * Night-mode-unaware-background-color

Tracking only

 * Large tables (buggy, not an error; for tracking only; not listed on Special page)

How you can help
Editors (mostly WikiGnomes) are going around Wikipedia working to clean up lint errors, which are sorted by severity into one of three priority levels: high, medium, and low, which relate to how badly the error affects page display, or how much the page display changed when MediaWiki parsing changed. You are welcome to join in this effort. Here are some hints:
 * Each lint error page has a help link in the upper-right corner that links to a page with more information about that type of error.
 * Lint error pages are sorted approximately in the order of the most recently edited being listed last. Some error pages are sorted better than others.
 * Lint error pages are not necessarily complete. When a new lint error type is discovered and a page is made for it, or when the definition of a type of lint error is changed, that lint error page starts empty and is gradually filled by a process that can take several weeks or months.
 * Every page's page information details how many errors of each type of lint error that page has. This section is near the end and is omitted if there are no lint errors.
 * For each lint error, the count maxes out at 20 in any one page.
 * It is OK to edit other people's User and User talk pages, and other people's comments on talk pages; but if you do, please see Wikipedia:Talk page guidelines § Editing others' comments for guidance.
 * Don't change the words of other editors.
 * Try to preserve the appearance.
 * It is OK to change the appearance in some cases if it preserves the original intent.
 * It is OK to fix a missing end tag, such as a tag improperly closed with another  tag instead of, even if this changes the appearance. This is especially true if the missing end tag affects anything beyond the scope of the comment in which it appears. If a user's comment in the middle of the page causes subsequent comments or sections to be indented wrong, or be bolded or italicized or in a different font, you should insert the missing end tag, even if the page has "always" been wrong.
 * ; after MediaWiki's July 2018 switch to a new linter package, many pages that used to look fine despite errors in them now show terrible appearance and accessibility problems, such as fonts becoming smaller and smaller (or larger and larger) the further down the page you scroll, due to successive unclosed sizing elements.
 * In a discussion about wiki or HTML markup, unclosed tags are sometimes used. For example, in a discussion about the tag, the tag might not be surrounded by o markup, so the  tag will be taken as markup with a missing end tag instead of simply displaying the tag. In cases like this, it is helpful to insert  around the unescaped markup, which changes the display, shows the intent of the original comment, and fixes the missing end tag or other errors resulting from the unescaped markup.
 * In a discussion about errors, for example, "Why does the display get messed up when I use [some bad markup]", it's often best to leave the bad markup in place, since otherwise the discussion won't make any sense.
 * Especially on User and User talk pages, try to minimize disruption by getting your fix right on the first try. "Show preview" is your friend.
 * By default, editing a base user talk page will trigger a notification to the user, which can be annoying and should not be done in large batches. To avoid this, use a flagged bot account, and also flag the edit as minor, which will bypass the "You have new messages" notification.
 * See also for a list of invalid tags and attributes, which you can detect with CSS. See below.
 * Some Lint errors caused by user signatures and Template substitutions are present across a large number of pages. It is more efficient to fix such errors in a bot task rather than manual edits. You can use regex-based insource search to identify patterns of errors that can be fixed by bots.
 * If you find a lint error in an article, consider the possibility that the error was introduced by a recent edit that should be reverted. This is especially true for Table tag that should be deleted and Fostered content lint errors, where careless deletion of table end markup can cause either of these lint errors. The solution to a lint error may be to revert one or more edits.
 * Occasionally, large pages show up on lint error lists without there actually being any errors on the pages themselves. If there's nothing obviously wrong with a listed page, and page information, lintHint, and template expansion show no errors, it will often disappear from the list on its own after a while. Editors can usually expedite this by null editing the page in question.

Reports

 * The Firefly Tools table, Outstanding linter errors on enwiki, is a chart with rows for the namespaces and columns for the type of lint error, with each cell in the chart listing the number of errors (maxed at 20 for each error type per article). This chart can help find a project of manageable size, or quickly check the number of lint errors of a certain type in a namespace, such as the Article namespace. This page is updated several times per hour.
 * Linter/reports/Articles by Lint Errors is a report of articles (i.e. pages in the article namespace) that have the most lint errors.
 * Linter/reports/Pages by Lint Errors is a similar report that covers pages in all namespaces. Note that the Linter error system tracks a maximum of 21 errors of any single type, so pages on this list may have more total errors than are shown in the report.
 * Linter/reports/Protected pages by Lint Errors, for protected pages by lint errors

Other useful pages

 * User:MalnadachBot/Signature submissions

Bots
Bots that are approved to run lint fixing tasks:
 * User:Legobot (Task 41)
 * User:MalnadachBot (Tasks 1, 2, 3, 5, 6, 7, 8, 9, 10, 11, 12 – blocked as of mid-2023)
 * User:Qwerfjkl (bot) (Tasks 27, 29)
 * User:SheepLinterBot (Tasks 1 and 2)
 * User:WOSlinkerBot (Tasks 1, 2, 4, 7, 8, 9, 10, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22)
 * User:WikiCleanerBot (Tasks 7, 10, 17, 22)

User Javascript tool: lintHint
User:PerfektesChaos/js/lintHint has instructions for installing and using lintHint, a gadget coded in JavaScript that identifies lint errors in a document in the wiki editor.

You can run lintHint repeatedly in the same edit session to see if you fixed the errors and to relocalize the error pointers. Error pointers are relative to the top of the article, so if you correct errors from the bottom up, you won't need to run lintHint again to relocalize error pointers.

The lintHint tool does not expand relative links when the page is in editing mode. For example, in Portal:Science,  really means , but lintHint does not do this. To get lintHint to work, you can manually expand relative links. You can also use Expand templates, and enter the page name in  and copy part or all of the page into. Then click  and then press. Expand templates will often help lintHint localize and identify lint errors listed on Page information but that lintHint doesn't find on its own.

After editing, pages are rechecked for lint errors, usually within seconds, but in the past sometimes delayed for hours. If lintHint says you fixed one or more lint errors, you probably did fix them, even if page information and the specific lint errors page aren't updated yet. As noted, however, lintHint can't detect errors in unexpanded relative links.

User CSS tool: lint.css
You can easily employ user CSS to detect a lot of "linty" old HTML 4 code in pages as you read, if you're a WikiGnome who likes to do cleanup. See meta:User:SMcCandlish/lint.css for a sample CSS declaration that makes various deprecated cruft – like, , , and – turn pink so it sticks out like a sore thumb. You can customize as you like for your own Special:MyPage/common.css or meta:Special:MyPage/global.css, or follow the instructions at lint.css to  (transclude) lint.css directly into your own user CSS at this or any other WMF wiki.

This CSS only detects no-longer-valid markup; it has no means of detecting other coding errors.

See here for another example.

Other errors

 * Linter/Pages with lint errors that should not be fixed
 * WikiProject Check Wikipedia – project devoted to this and other types of Wikipedia code cleanup
 * (some might be valid usage denoting a bold phrase inside single quote marks, see MOS:SINGLE.)
 * (some might be valid usage denoting a bold phrase inside single quote marks, see MOS:SINGLE.)

Help pages

 * – simplified instructions for fixing pages for the modern MediaWiki parser
 * Help:HTML in wikitext
 * HTML5 – information page on technical details of updating WP code to HTML5 + CSS3, including how to replace deprecated HTML 4.01 markup; includes automated searches for obsolete markup
 * Manual of Style/Accessibility
 * WikiProject Accessibility