The {{deprecated code}} template (easiest used from its {{dc}} redirect) can be used to indicate, e.g. in template documentation or Wikipedia articles on things like HTML specifications, code that has been deprecated and should not normally be used. On the technical level it is a CSS-styled <del>...</del> that greys out the text (the near-universal sign in computing and computing documentation for "doesn't work", "don't do this", "bad code", "ignore", "option not available", etc.), and removes that element's usual strikethrough (CSS: "line-through") rendering, which makes the content hard to read. If you really want that line-through, use the {{dc2}} variant to do this; it is otherwise identical.
|2= or |title= a mouse-over "tooltip" (in some browsers, anyway), e.g. for briefly explaining the deprecation, e.g. "Deprecated since HTML 3.0" or "Breaks infobox formatting"
|red=y make the text red instead of grey, for indicating dangerous/error/forbidden things (a shortcut for this is {{dcr}})
|class= assign a CSS class
|id= assign an HTML ID for #linking and other purposes (must be unique on the page and start with an alphabetic letter)
|style= add additional CSS styling (can be used to add text-decoration:line-through; back in if you want that formatting; a shortcut for this is {{dc2}})
To indicate text is a variable name. Use for any variable names except those including "I" (uppercase i) and/or "l" (lowercase L); for these, {{varserif}} should be used to ensure a noticeable distinction.
To display parameters as used in code (i.e. with triple braces), especially to indicate relationships between them. May be combined with {{para}} above.
() To indicate deprecated source code in template documentation, articles on HTML specs, etc. The {{dc2}} variant uses strike-through (<blink>) while {{dc|red{{=}}y}} uses red (<blink>).
