While HTML is used for defining the content structure of a web
document or a web user interface, the Cascading Style Sheets (CSS) language is used for defining the
style of web pages, which means that you use it for
telling the browser how you want your HTML (or XML) rendered: using which
layout of content elements, which fonts and text styles, which colors, which
backgrounds, and which animations. Normally, these settings are made in a
separate CSS file that is associated with an HTML file via a special
link element in the HTML's
CSS is based on a form of rules that consist of selectors, which select the document element(s) to
which a rule applies, and a list of property-value
pairs that define the styling of the selected element(s) with the
help of CSS properties such as
There are two fundamental mechanisms for computing the CSS property values
for any page element as a result of applying the given set of CSS rules:
inheritance and the cascade.
The basic element of a CSS layout is a rectangle, also called "box", with an inner content area, an optional border, an optional padding (between content and border) and an optional margin around the border. This structure is defined by the CSS box model.
We will not go deeper into CSS in this book, since our focus here is on the logic and functionality of an app, and not so much on its beauty.