CSS stands for Cascading Style Sheets
Development of CSS:
CSS was first proposed by Håkon Wium Lie on October 10, 1994 at the time, Lie was working with Tim Berners-Lee at CERN. The first W3C CSS Recommendation (CSS1) being released in 1996. In particular, a proposal by Bert Bos was influential; he became co-author of CSS1, and is regarded as co-creator of CSS.
CSS has different levels and profiles. Each level of CSS builds upon the last, typically adding new features and typically termed as CSS 1, CSS 2, CSS 3, and CSS 4. Profiles are typically a subset of one or more levels of CSS built for a particular device media or user interface. Currently there are profiles for mobile devices, printers, and television sets. Profiles should not be confused with media types, which were added in CSS 2
The first CSS specification to become an official W3C Recommendation is CSS level 1, published on December 17, 1996. Håkon Wium Lie and Bert Bos are credited as the original developers.
CSS level 2 determination was created by the W3C and distributed as a suggestion in May 1998. A superset of CSS 1, CSS 2 incorporates various new capacities like total, relative, and fixed positioning of components and z-index, the idea of media types, support for aural style sheet (which were later supplanted by the CSS 3 speech modules) and bidirectional text, and new font properties, for example, shadows.
CSS level 2 revision 1, often referred to as "CSS 2.1", fixes errors in CSS 2, expels inadequately supported or not fully interoperable features and adds already implemented browser extensions to the specification. To comply with the W3C Process for standardizing technical specifications, CSS 2.1 came back and forth between Working Draft status and Candidate Recommendation status for a long time.
Unlike CSS 2, which is a large single specification defining different features, CSS 3 is splits into many separate documents called "modules". Each module adds new capabilities or extends features defined in CSS 2, safeguarding backward compatibility. Work on CSS level 3 began around the time of publication of the original CSS 2 recommendation. The first CSS 3 drafts were published in June 1999.
There is no single, incorporated CSS4 specification, because it is split into separate "level 4" modules.
Since CSS3 split the CSS language's definition into modules, the modules have been permitted to level autonomously. Most modules are level 3—they build on things from CSS 2.1. A couple of level-4 modules exist (for example, Image Values, Backgrounds and Borders, or Selectors),which build on the functionality of a former level-3 module. Different modules defining completely new functionality, such as Flexbox, have been assigned as "level 1".
The CSS Working Group sometimes publishes "Snapshots", an accumulation of entire modules and parts of other drafts that are considered stable, interoperably implemented and hence ready to utilize. Up until this point, five such "best current practices" documents have been published as Notes, in 2007, 2010, 2015, 2017, and 2018.
A CSS rule-set consists of a selector and a declaration block:
The selector points to the HTML element you want to style.
The declaration block contains one or more declarations separated by semicolons.
Each declaration includes a CSS property name and a value, separated by a colon.
A CSS declaration always ends with a semicolon, and declaration blocks are surrounded by curly braces.