Quirks mode

HTML

HTML and variants
Dynamic HTML HTML5 XHTML Basic Mobile Profile
HTML elements and attributes
HTML element article audio blink canvas div and span marquee meta video HTML attribute alt attribute HTML frame
Editing
HTML editor Text editor
Character encodings and language
Character encodings Character entity references (named characters) Unicode Language code
Document and browser models
Document Object Model Browser Object Model Style sheets CSS Font family Web colors
Client-side scripting and APIs
JavaScript WebCL HTMX
Graphics and Web3D technology
Web3D WebGL WebGPU WebXR W3C Validator WHATWG Quirks mode Web storage Rendering engine
Comparisons
Document markup languages Comparison of browser engines

In computing, quirks mode is an approach used by web browsers to maintain backward compatibility with web pages designed for old web browsers, instead of strictly complying with web standards in standards mode. This behavior has since been codified, so what was previously standards mode is now referred to as simply no quirks mode.