HTML/CSS is as popular as the World Wide Web because it is used in all websites and apps all over the world. The first point of contact of the World Wide Web are HTML and the CSS. They are not very interactive as other programming languages.  They are used to design the look and flair of the website.  The appearance of your website is determined by the HTML and CSS.

It is only very large organisations that engage HTML/CSS engineers all over the world, because you will be required to learn at leat one other programming language with it.  Programmers except backend developers are required to learn HTML/CSS.

Content

Introduction
Syntax
Types of CSS
Inline Styles
Embedded Styles
External Style Sheets
Importing External Styles
CSS Selectors
CSS Simple Selectors
Selector lists
Attribute selectors
Pseudo-classes and elements
Universal Selector (*)
CSS Color
CSS HEX Value
RGB Colors
RGBA Value
HSL Value
CSS Background
Opacity
RGBA Transparency
CSS Border
Border Properties
Border Color
Border-width
Border Radius
CSS Fonts
Font-family
Font Style
Font Size
Font Variant
CSS Text
Text Color
Text Alignment
CSS Links
link with states
Creating link buttons with CSS
Custom Color Links
CSS Lists
Unordered Lists
List Markers
CSS Navigation bar
HTML Tables
Zebra-striped Tables
CSS Padding
Padding with two properties
Padding with one property
CSS Margin
The margin for individual sides
The margin for All Sides