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. Ambient Light Sensor

    Defines a concrete sensor interface to monitor the ambient light level or illuminance of the device’s environment.

  3. Client Hints: DPR, Width, Viewport-Width

    DPR, Width, and Viewport-Width hints enable proactive content negotiation between client and server, enabling automated delivery of optimized assets - e.g. auto-negotiating image DPR resolution.

  4. Synchronous Clipboard API

    API to provide copy, cut and paste events as well as provide access to the OS clipboard.

  5. CSS :any-link selector

    The `:any-link` CSS pseudo-class matches all elements that match `:link` or `:visited`

  6. CSS first-line pseudo-element

    Allows styling specifically for the first line of text using the `::first-line` pseudo-element. Note that only a limited set of properties can be applied.

  7. CSS line-clamp

    CSS property that will contain text to a given amount of lines when used in combination with `display: -webkit-box`. It will end with ellipsis when `text-overflow: ellipsis` is included.

  8. selector list argument of :not()

    Selectors Level 3 only allowed `:not()` pseudo-class to accept a single simple selector, which the element must not match any of. Thus, `:not(a, .b, [c])` or `:not(a.b[c])` did not work. Selectors Level 4 allows `:not()` to accept a list of selectors. Thus, `:not(a):not(.b):not([c])` can instead be written as `:not(a, .b, [c])` and `:not(a.b[c])` works as intended.

  9. selector list argument of :nth-child and :nth-last-child CSS pseudo-classes

    The newest versions of `:nth-child()` and `:nth-last-child()` accept an optional `of S` clause which filters the children to only those which match the selector list `S`. For example, `:nth-child(1 of .foo)` selects the first child among the children that have the `foo` class (ignoring any non-`foo` children which precede that child). Similar to `:nth-of-type`, but for arbitrary selectors instead of only type selectors.

  10. 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.

  11. HTTP Live Streaming (HLS)

    HTTP-based media streaming communications protocol

  12. Offline web applications

    Now deprecated method of defining web page files to be cached using a cache manifest file, allowing them to work offline on subsequent visits to the page.

  13. 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.

  14. "once" event listener option

    Causes an event listener to be automatically removed after it gets invoked, so that it only gets invoked once. Similar to jQuery's `$.one()` feature.

  15. CSS outline properties

    The CSS outline properties draw a border around an element that does not affect layout, making it ideal for highlighting. This covers the `outline` shorthand, as well as `outline-width`, `outline-style`, `outline-color` and `outline-offset`.

  16. Passive event listeners

    Event listeners created with the `passive: true` option cannot cancel (`preventDefault()`) the events they receive. Primarily intended to be used with touch events and `wheel` events. Since they cannot prevent scrolls, passive event listeners allow the browser to perform optimizations that result in smoother scrolling.

  17. Link type "noreferrer"

    Links with `rel="noreferrer"` set do not send the request's "referrer" header. This prevents the destination site from seeing what URL the user came from.

  18. ES6 Template Literals (Template Strings)

    Template literals are string literals allowing embedded expressions using backtick characters (`). You can use multi-line strings and string interpolation features with them. Formerly known as template strings.

  19. text-decoration styling

    Method of defining the type, style and color of lines in the text-decoration property. These can be defined as shorthand (e.g. `text-decoration: line-through dashed blue`) or as single properties (e.g. `text-decoration-color: blue`)

  20. document api: linkcolor

  21. document api: links

  22. fontface api: linegapoverride

  23. linearaccelerationsensor api

  24. path2d api: lineto

  25. vrpose api: linearacceleration

  26. vrpose api: linearvelocity

  27. vttcue api: line

  28. vttregion api: lines

  29. xrpose api: linearvelocity

  30. html element: li

  31. html element: link

  32. html element: li: type

  33. headers http header: link

  34. javascript built-in: intl: listformat: format

  35. javascript built-in: intl: listformat: formattoparts

  36. javascript built-in: intl: listformat: `listformat()` constructor

  37. javascript built-in: intl: listformat: resolvedoptions

  38. javascript built-in: intl: listformat: supportedlocalesof

  39. javascript built-in: string: link

  40. grammar: array literals (`[1, 2, 3]`)

  41. grammar: boolean literals (`true`/`false`)

  42. grammar: null literal (`null`)

  43. grammar: regular expression literals (`/ab+c/g`)

  44. grammar: string literals (`'hello world'`)

  45. svg element: line

  46. svg element: lineargradient

  47. svg element: line: x1

  48. svg element: line: x2

  49. svg element: line: y1

  50. svg element: line: y2