Autofocus attribute

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

contenteditable attribute (basic support)

Method of making any HTML element editable.

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">`.

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)

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).

dataset & data-* attributes

Method of applying and accessing custom data to elements.

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.

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.

Form attribute

Attribute for associating input and submit buttons with a form.

Attributes for form submission

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

hidden attribute

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

sandbox attribute for iframes

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

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.

srcdoc attribute for iframes

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

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.

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

inputmode attribute

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

Minimum length attribute for input fields

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

Pattern attribute for input fields

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

input placeholder attribute

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

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.

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.

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.

Ping attribute

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

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.

'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.

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.

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.

Spellcheck attribute

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

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).

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).

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.

attr api

attr api: localname

attr api: name

attr api: namespaceuri

attr api: ownerelement

attr api: prefix

attr api: specified

attr api: value

html attribute: class

html attribute: dir

html attribute: id

html attribute: inert

html attribute: is

html attribute: lang

html attribute: nonce

html attribute: part

html attribute: slot

html attribute: title