CSSStyleDeclaration represents a collection of CSS property-value pairs. It is used in a few APIs:
The CSSRule.selectorText property gets the textual representation of the selector for the rule set. This is implemented in a readonly manner; to set stylesheet rules dynamically, see Using dynamic styling information.
CSSStyleSheet.insertRule() method inserts a new CSS rule into the current style sheet. (with some restrictions)
Enables the style sheets matching the specified name in the current style sheet set, and disables all other style sheets (except those without a title, which are always enabled).
Returns the preferred style sheet set as set by the page author.
The Element.clientHeight read-only property is zero for elements with no CSS or inline layout boxes, otherwise it's  the inner height of an element in pixels, including padding but not the horizontal scrollbar height, border, or margin.
The width of the left border of an element in pixels. It includes the width of the vertical scrollbar if the text direction of the element is right–to–left and if there is an overflow causing a left vertical scrollbar to be rendered. clientLeft does not include the left margin or the left padding. clientLeft is read-only.
The width of the top border of an element in pixels. It does not include the top margin or padding. clientTop is read-only.
The Element.clientWidth property is zero for elements with no CSS or inline layout boxes, otherwise it's the inner width of an element in pixels. It includes padding but not the vertical scrollbar (if present, if rendered), border or margin.
The Element.getClientRects() method returns a collection of rectangles that indicate the bounding rectangles for each box in a client.
The Element.scrollIntoView() method scrolls the current element into the visible area of the browser window.
The Element.scrollLeft property gets or sets the number of pixels that an element's content is scrolled to the left.
The Element.scrollTop property gets or sets the number of pixels that the content of an element is scrolled upward. An element's scrollTop is a measurement of the distance of an element's top to its topmost visible content. When an element content does not generate a vertical scrollbar, then its scrollTop value defaults to 0.
The Element.scrollWidth read–only property returns either the width in pixels of the content of an element or the width of the element itself, whichever is greater. If the element is wider than its content area (for example, if there are scroll bars for scrolling through the content), the scrollWidth is larger than the clientWidth.
The HTMLElement.offsetHeight read-only property is the height of the element including vertical padding and borders, in pixels, as an integer.