What are “Classes” in CSS?

The .class selector selects elements with a specific class attribute. To select elements with a specific class, write a period (.) character, followed by the name of the class:
   .intro {background-color: yellow;}
You can also specify that only specific HTML elements should be affected by a class. To do this, start with the element name, then write the period (.) character, followed by the name of the class:
   p.hometown {background color: yellow;}
HTML elements can also refer to more than one class:
   <p class="center large">This paragraph refers to two classes.

What are 3 ways to attach a stylesheet to html?

External Style Sheet

With an external style sheet, you can change the look of an entire website by changing just one file. Each page must include a reference to the external style sheet file inside the <link> element. The <link> element goes inside the <head> section:
   <link rel="stylesheet" type="text/css"
href="mystyle.css"> </head>

Internal Style Sheet

An internal style sheet may be used if one single page has a unique style. Internal styles are defined within the <style> element, inside the <head> section of an HTML page:
   <head> <style> body {background-color: linen;} h1 {color: maroon;    margin-left: 40px;}  </style> </head>

Inline Styles

An inline style may be used to apply a unique style for a single element. To use inline styles, add the style attribute to the relevant element. The style attribute can contain any CSS property. The example below shows how to change the color and the left margin of a <h1> element:
   <h1 style="color:blue;margin-left:30px;">This is a heading</h1>

What is cascade in CSS?

"Cascading" means that styles can fall (or cascade) from one style sheet to another, enabling multiple style sheets to be used on one HTML document.

Cascading Order

What style will be used when there is more than one style specified for an HTML element? Generally speaking we can say that all the styles will "cascade" into a new "virtual" style sheet by the following rules, where number one has the highest priority:

  1. Inline style (inside an HTML element)
  2. External and internal style sheets (in the head section)
  3. Browser default

So, an inline style (inside a specific HTML element) has the highest priority, which means that it will override a style defined inside the <head> tag, or in an external style sheet, or a browser default value.