1. CSS ::marker pseudo-element

    The `::marker` pseudo-element allows list item markers to be styled or have their content value customized.

  2. Custom Elements (deprecated V0 spec)

    Original V0 version of the custom elements specification. See [Custom Elements V1](#feat=custom-elementsv1) for support for the latest version.

  3. Custom Elements (V1)

    One of the key features of the Web Components system, custom elements allow new HTML tags to be defined.

  4. Customizable Select element

    Customization of the `<select>` element's appearance. Previously envisioned as a separate element (`<selectlist>` or `<selectmenu>`).

  5. Datalist element

    Method of setting a list of options for a user to select in a text field, while leaving the ability to enter a custom value.

  6. dataset & data-* attributes

    Method of applying and accessing custom data to elements.

  7. Dialog element

    Method of easily creating custom dialog boxes to display to the user with modal or non-modal options. Also includes a `::backdrop` pseudo-element for behind the element.

  8. customelementregistry api

  9. customelementregistry api: customized built-in element support

  10. customelementregistry api: `customelementregistry()` constructor

  11. customelementregistry api: define

  12. customelementregistry api: define: `connectedmovecallback()` lifecycle callback

  13. customelementregistry api: define: `disabledfeatures` static property

  14. customelementregistry api: define: emoji and more unicode in element and attribute names and namespace prefix (valid names match html parser)

  15. customelementregistry api: get

  16. customelementregistry api: getname

  17. customelementregistry api: initialize

  18. customelementregistry api: upgrade

  19. customelementregistry api: whendefined

  20. customelementregistry api: whendefined: emoji and more unicode in element and attribute names and namespace prefix (valid names match html parser)

  21. document api: createelementns: `options` parameter: `customelementregistry` option

  22. document api: createelement: `options` parameter: `customelementregistry` option

  23. document api: customelementregistry

  24. element api: customelementregistry

  25. htmlbuttonelement api: setcustomvalidity

  26. htmlfieldsetelement api: setcustomvalidity

  27. htmlinputelement api: setcustomvalidity

  28. htmlobjectelement api: setcustomvalidity

  29. htmloutputelement api: setcustomvalidity

  30. htmlselectelement api: setcustomvalidity

  31. htmltemplateelement api: shadowrootcustomelementregistry

  32. htmltextareaelement api: setcustomvalidity

  33. shadowroot api: customelementregistry

  34. window api: customelements

  35. html element: iframe: sandbox: `sandbox="allow-top-navigation-to-custom-protocols"`

  36. html element: template: shadowrootcustomelementregistry