The modern standard API for adding DOM event handlers. Introduced in the DOM Level 2 Events spec. Also implies support for `removeEventListener`, the [capture phase](https://dom.spec.whatwg.org/#dom-event-capturing_phase) of DOM event dispatch, as well as the `stopPropagation()` and `preventDefault()` event methods.
Method of defining how a background image is attached to a scrollable element. Values include `scroll` (default), `fixed` and `local`.
Battery Status API
Method to provide information about the battery status of the hosting device.
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">`.
::first-letter CSS pseudo-element selector
CSS pseudo-element that allows styling only the first "letter" of text within an element. Useful for implementing initial caps or drop caps styling.
The font-palette CSS property allows selecting a palette from a color font. In combination with the `@font-palette-values` at-rule, custom palettes can be defined.
CSS Initial Letter
Method of creating an enlarged cap, including a drop or raised cap, in a robust way.
letter-spacing CSS property
Controls spacing between characters of text (i.e. "tracking" in typographical terms). Not to be confused with kerning.
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).
Method to programmatically trigger a DOM event.
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.
Method of applying advanced typographic and language-specific font features to supported OpenType fonts.
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`
The `hidden` attribute may be applied to any element, and effectively hides elements similar to `display: none` in CSS.
HTTP Live Streaming (HLS)
HTTP-based media streaming communications protocol
Networking protocol for low-latency transport of content over the web. Originally started out from the SPDY protocol, now standardized as HTTP version 2.
Third version of the HTTP networking protocol which uses QUIC as transport protocol. Previously known as HTTP-over-QUIC, now standardized as HTTP/3.
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.
The `inputmode` attribute specifies what kind of input mechanism would be most helpful for users entering content into the form control.
Pattern attribute for input fields
Allows validation of an input field based on a given regular expression pattern.
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.
When used on an anchor, this attribute signifies that the browser should send a ping request the resource the attribute points to.
HTTP Public Key Pinning
Declare that a website's HTTPS certificate should only be treated as valid if the public key is contained in a list specified over HTTP to prevent MITM attacks that use valid CA-issued certificates.
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
defer attribute for external scripts
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).
Signed HTTP Exchanges (SXG)
Part of the Web Packaging spec, Signed HTTP Exchanges allow a different origin server to provide a resource, and this will be treated as if it came from the original server. This can be used with AMP CDNs, for example, to allow the original URL to be displayed in the URL bar.
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.
TTF/OTF - TrueType and OpenType font support
Support for the TrueType (.ttf) and OpenType (.otf) outline font formats in @font-face.
WebVTT - Web Video Text Tracks
Format for marking up text captions for multimedia resources.
XMLHttpRequest advanced features
Adds more functionality to XHR (aka AJAX) requests like file uploads, transfer progress information and the ability to send form data. Previously known as [XMLHttpRequest Level 2](https://www.w3.org/TR/2012/WD-XMLHttpRequest-20120117/), these features now appear simply in the XMLHttpRequest spec.
html element: tt
http method: connect
http method: delete
http method: get
http method: head
http method: options
http method: post
http method: put
http status code: 204
http status code: 206
http status code: 301