Class List
Here are the classes, structs, unions and interfaces with brief descriptions:

OOUI |
AccessKeyedElement | Element with an accesskey
ActionFieldLayout | Layout made of a field, button and optional label
ApexTheme |
ButtonElement | Element with a button
ButtonGroupWidget | Group widget for multiple related buttons
ButtonInputWidget | A button that is an input widget
ButtonWidget | Generic widget for buttons
CheckboxInputWidget | Checkbox input widget
ComboBoxInputWidget | Combo box input widget, wrapping a text input with `<datalist>`
DropdownInputWidget | Dropdown input widget, wrapping a `<select>` element
Element | DOM element abstraction
ElementMixin |
Exception |
FieldLayout | Layout made of a field and optional label
FieldsetLayout | Layout made of a fieldset and optional legend
FlaggedElement | Element with named flags that can be added, removed, listed and checked
FormLayout | Layout with an HTML form
GroupElement | Element containing a sequence of child elements
HorizontalLayout | HorizontalLayout arranges its contents in a single line (using `display: inline-block` for its items), with small margins between them
HtmlSnippet | Wraps a HTML snippet for use with Tag::appendContent() and Tag::prependContent()
IconElement | Element containing an icon
IconWidget | Icon widget
IndicatorElement | Element containing an indicator
IndicatorWidget | Indicator widget
InputWidget | Base class for input widgets
LabelElement | Element containing a label
LabelWidget | Label widget
Layout | Container for elements
MediaWikiTheme |
PanelLayout | Layout that expands to cover the entire area of its parent, with optional scrolling and padding
RadioInputWidget | Radio input widget
RadioSelectInputWidget | Multiple radio buttons input widget
TabIndexedElement | Element supporting "sequential focus navigation" using the 'tabindex' attribute
Tag |
TextInputWidget | Input widget with a text field
Theme | Theme logic
TitledElement | Element with a title
Widget | User interface control