The HTML <multicol> element was an experimental element designed to allow multi-column layouts. It never got any significant traction and is not implemented in any major browsers.
The generator comprehension syntax is a JavaScript expression which allows you to quickly assemble a new generator function based on an existing iterable object. Comprehensions exist in many programming languages.
The ::-moz-progress-bar CSS pseudo-element applies to the area of an HTML <progress> element that represents the amount of progress that has happened so far. This lets you, for example, change the color of progress bars.
The ::-ms-fill CSS pseudo-element represents the filled-in portion of a <progress> element. This pseudo-element is non-standard and specific to Internet Explorer 10+, hence the vendor prefix.
The ::-webkit-progress-bar CSS pseudo-element represents the entire bar of a <progress> element. Normally it's only visible as the unfilled portion of the bar, since by default it's rendered below the ::-webkit-progress-value pseudo-element. It is a child of the ::-webkit-progress-inner-element pseudo-element and the parent of the ::-webkit-progress-value pseudo-element.
The ::-webkit-progress-inner-element CSS pseudo-element represents the outermost, container pseudo-element of the <progress> element. It is the parent of the ::-webkit-progress-bar pseudo-element.
The ::-webkit-progress-value CSS pseudo-element represents the filled-in portion of the bar of a <progress> element. It is a child of the ::-webkit-progress-bar pseudo-element.
The :-moz-broken CSS pseudo-class matches elements representing broken image links.
The :-moz-drag-over CSS pseudo-class is used to edit an element when a drag-over event is called on it.
The :-moz-first-node CSS pseudo-class represents any element that is the first child node of some other element. It differs from :first-child because it does not match a first child element with (non-whitespace) text before it.
The :-moz-focusring CSS pseudo-class is similar to the :focus pseudo-class, but it only matches an element if the element is currently focused and a focus ring or other indicator should be drawn for that element. If :-moz-focusring matches, then :focus also matches, but the converse is not always true - it depends on whether the user agent has focus ring drawing enabled and how the element was focused. Whether the user agent has focus ring drawing enabled can depend on things like the settings of the operating system the user is using, so the precise behavior of this pseudo-class can vary from platform to platform depending on each platforms' particular focus best practices (defaults) or user modified settings.
The :-moz-last-node CSS pseudo-class matches an element that is the last child node of some other element. It differs from :last-child because it does not match a last child element with (non-whitespace) text after it.
The non-standard ::-moz-list-bullet Mozilla CSS pseudo-element is used to style the bullet of a list element.
The :-moz-lwtheme-darktext pseudo-class matches in chrome documents when :-moz-lwtheme is true and a lightweight theme with a dark text color is selected.
:-moz-system-metric(mac-graphite-theme) will match an element if the user has chosen the "Graphite" appearance in the "Appearance" prefpane of the Mac OS X System Preferences.
The :-webkit-autofill CSS pseudo-class matches when an <input> element has its value autofilled by the browser.
In Mozilla applications like Firefox, the -moz-border-bottom-colors CSS property sets a list of colors for the bottom border.