The z-index helps to specify the stack order of positioned elements that may overlap one another. The z-index default value is zero and can take on either a positive or negative number.

An element with a higher z-index is always stacked above than a lower index.

Z-Index can take the following values:

• Auto: Sets the stack order equal to its parents.
• Number: Orders the stack order.
• Initial: Sets this property to its default value (0).
• Inherit: Inherits this property from its parent element.

visibility: hidden hides the element, but it occupies space and affects the layout of the document.

display: none also hides the element but not occupy space. It will not affect the layout of the document.

W3C stands for World Wide Web Consortium. Its purpose is to deliver the information of the World Wide Web. It also develops rules and guidelines for the Web.

It is the process of generating intermediate frames between two images.

It gives the impression that the first image has smoothly evolved into the second one.

It is an important method used in all types of animations.

In CSS3, Transforms (matrix, translate, rotate, scale) module can be used to achieve tweening.

The main difference between CSS2 and CSS3 is that CSS3 is divided into different sections which are also known as modules. Unlike CSS2, CSS3 modules are supported by many browsers.

Apart from that, CSS3 contains new General Sibling Combinators which is responsible for matching the sibling elements with the given elements.