1. EventTarget.addEventListener()

    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.

  2. CSS background-attachment

    Method of defining how a background image is attached to a scrollable element. Values include `scroll` (default), `fixed` and `local`.

  3. Battery Status API

    Method to provide information about the battery status of the hosting device.

  4. 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">`.

  5. ::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.

  6. CSS font-palette

    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.

  7. CSS Initial Letter

    Method of creating an enlarged cap, including a drop or raised cap, in a robust way.

  8. letter-spacing CSS property

    Controls spacing between characters of text (i.e. "tracking" in typographical terms). Not to be confused with kerning.

  9. 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).

  10. EventTarget.dispatchEvent

    Method to programmatically trigger a DOM event.

  11. 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.

  12. 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.

  13. CSS font-feature-settings

    Method of applying advanced typographic and language-specific font features to supported OpenType fonts.

  14. Form attribute

    Attribute for associating input and submit buttons with a form.

  15. Attributes for form submission

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

  16. hidden attribute

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

  17. HTTP Live Streaming (HLS)

    HTTP-based media streaming communications protocol

  18. HTTP/2 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.

  19. HTTP/3 protocol

    Third version of the HTTP networking protocol which uses QUIC as transport protocol. Previously known as HTTP-over-QUIC, now standardized as HTTP/3.

  20. 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.

  21. srcdoc attribute for iframes

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

  22. inputmode attribute

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

  23. Pattern attribute for input fields

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

  24. 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.

  25. 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.

  26. 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.

  27. Ping attribute

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

  28. 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.

  29. 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.

  30. '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.

  31. 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.

  32. 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.

  33. Spellcheck attribute

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

  34. 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).

  35. 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.

  36. 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.

  37. TTF/OTF - TrueType and OpenType font support

    Support for the TrueType (.ttf) and OpenType (.otf) outline font formats in @font-face.

  38. WebVTT - Web Video Text Tracks

    Format for marking up text captions for multimedia resources.

  39. 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.

  40. html element: tt

  41. http method: connect

  42. http method: delete

  43. http method: get

  44. http method: head

  45. http method: options

  46. http method: post

  47. http method: put

  48. http status code: 204

  49. http status code: 206

  50. http status code: 301