1. Autofocus attribute

    Allows a form field to be immediately focused on page load.

  2. contenteditable attribute (basic support)

    Method of making any HTML element editable.

  3. Case-insensitive CSS attribute selectors

    Including an `i` before the `]` in a CSS attribute selector causes the attribute value to be matched in an ASCII-case-insensitive manner. For example, `[b="xyz" i]` would match both `<a b="xyz">` and `<a b="XYZ">`.

  4. CSS 2.1 selectors

    Basic CSS selectors including: `*` (universal selector), `>` (child selector), `:first-child`, `:link`, `:visited`, `:active`, `:hover`, `:focus`, `:lang()`, `+` (adjacent sibling selector), `[attr]`, `[attr="val"]`, `[attr~="val"]`, `[attr|="bar"]`, `.foo` (class selector), `#foo` (id selector)

  5. CSS3 attr() function for all properties

    While `attr()` is supported for effectively all browsers for the `content` property, CSS Values and Units Level 5 adds the ability to use `attr()` on **any** CSS property, and to use it for non-string values (e.g. numbers, colors).

  6. dataset & data-* attributes

    Method of applying and accessing custom data to elements.

  7. Download attribute

    When used on an anchor, this attribute signifies that the browser should download the resource the anchor points to rather than navigate to it.

  8. disabled attribute of the fieldset element

    Allows disabling all of the form control descendants of a fieldset via a `disabled` attribute on the fieldset element itself.

  9. Form attribute

    Attribute for associating input and submit buttons with a form.

  10. Attributes for form submission

    Attributes for form submission that may be specified on submit buttons. The attributes are: `formaction`, `formenctype`, `formmethod`, `formnovalidate`, and `formtarget`

  11. hidden attribute

    The `hidden` attribute may be applied to any element, and effectively hides elements similar to `display: none` in CSS.

  12. sandbox attribute for iframes

    Method of running external site pages with reduced privileges (e.g. no JavaScript) in iframes.

  13. seamless attribute for iframes

    The seamless attribute makes an iframe's contents actually part of a page, and adopts the styles from its hosting page. The attribute has been removed from both [the WHATWG](https://github.com/whatwg/html/issues/331) and [the W3C](https://github.com/w3c/html/pull/325) HTML5 specifications.

  14. srcdoc attribute for iframes

    Override the content specified in the `src` attribute (if present) with HTML content within the attribute.

  15. autocomplete attribute: on & off values

    The `autocomplete` attribute for `input` elements indicates to the browser whether a value should or should not be autofilled when appropriate.

  16. accept attribute for file input

    Allows a filter to be defined for what type of files a user may pick with from an `<input type="file">` dialog

  17. inputmode attribute

    The `inputmode` attribute specifies what kind of input mechanism would be most helpful for users entering content into the form control.

  18. Minimum length attribute for input fields

    Declares a lower bound on the number of characters a user can input.

  19. Pattern attribute for input fields

    Allows validation of an input field based on a given regular expression pattern.

  20. input placeholder attribute

    Method of setting placeholder text for text-like input fields, to suggest the expected inserted information.

  21. Lazy loading via attribute for images & iframes

    The `loading` attribute on images & iframes gives authors control over when the browser should start loading the resource.

  22. maxlength attribute for input and textarea elements

    Declares an upper bound on the number of characters the user can input. Normally the UI ignores attempts by the user to type in additional characters beyond this limit.

  23. Reversed attribute of ordered lists

    This attribute makes an ordered list number its items in descending order (large to small), instead of ascending order (small to large; the default). The order that the list items are displayed in is not affected.

  24. Ping attribute

    When used on an anchor, this attribute signifies that the browser should send a ping request the resource the attribute points to.

  25. readonly attribute of input and textarea elements

    Makes the form control non-editable. Unlike the `disabled` attribute, `readonly` form controls are still included in form submissions and the user can still select (but not edit) their value text.

  26. 'SameSite' cookie attribute

    Same-site cookies ("First-Party-Only" or "First-Party") allow servers to mitigate the risk of CSRF and information leakage attacks by asserting that a particular cookie should only be sent with requests initiated from the same registrable domain.

  27. async attribute for external scripts

    The boolean async attribute on script elements allows the external JavaScript file to run when it's available, without delaying page load first.

  28. defer attribute for external scripts

    The boolean defer attribute on script elements allows the external JavaScript file to run when the DOM is loaded, without delaying page load first.

  29. Spellcheck attribute

    Attribute for `input`/`textarea` fields to enable/disable the browser's spellchecker.

  30. Srcset and sizes attributes

    The `srcset` and `sizes` attributes on `img` (or `source`) elements allow authors to define various image resources and "hints" that assist a user agent to determine the most appropriate image source to display (e.g. high-resolution displays, small monitors, etc).

  31. Scoped attribute

    Deprecated method of allowing scoped CSS styles using a "scoped" attribute. Now [removed from the specification](https://github.com/whatwg/html/issues/552) and replaced by the [@scope CSS rule](/css-cascade-scope).

  32. tabindex global attribute

    Specifies the focusability of the element and in what order (if any) it should become focused (relative to other elements) when "tabbing" through the document.

  33. attr api

  34. attr api: localname

  35. attr api: name

  36. attr api: namespaceuri

  37. attr api: ownerelement

  38. attr api: prefix

  39. attr api: specified

  40. attr api: value

  41. html attribute: class

  42. html attribute: dir

  43. html attribute: id

  44. html attribute: inert

  45. html attribute: is

  46. html attribute: lang

  47. html attribute: part

  48. html attribute: slot

  49. html attribute: style

  50. html attribute: title