Cascading Style Sheets are controls for the design of web pages. They are kind of like styles in MS Word. For example you can specify that all level 1 headings (HTML code <H1>) will be 16 point bold red with a helvetica font family. Styles can refer to a whole web site, just a page or a section within a page.
HTML provides structure to a document specifying headers, paragraphs, lists, tables, ...
CSS

<P STYLE="font-family: Arial, Verdana, Sans-serif; font-weight:bold;
font-size: 16pt;  text-align: center; color:red;">Call 911 for all emergencies</P>

Units:
e.g. ul {margin-left: 15px;
    padding-left: 15px;}

    * in is inches
    * cm is centimetres
    * mm is millimetres
    * pc is picas
    * ex is x-height
Keywords:
 e.g. b [font-size:large}
    * xx-small
    * x-small
    * small
    * medium
    * large
    * x-large
    * xx-large
    
    <STYLE type="text/css">
LI {font-family: Arial, Verdana, Sans-serif; font-size: 12pt; padding:4;}
.new {background-color:green;}
TH {font-family:arial,helvetica; font-size: 10pt;}
</STYLE>

<P STYLE="font-family: Times New Roman, serif; font-weight:bold;
font-size: 16pt;  text-align: center; white-space:nowrap;">


<ul STYLE="list-style-type: circle;">
disk, square, circle, armenian

<SPAN STYLE="font-size: 1pt; color:#FFFFCC;">Invisable</SPAN>

<TD STYLE="color:white;
  background-color:#6A5ACD;">

ul {margin-left: 15px;
    padding-left: 15px;}
<ul STYLE="list-style-image: url('../../images/dash.gif');">
<ul STYLE="list-style-type: none;"><li><b>-</b> xxx

<STYLE type="text/css">
.style0 {text-align:general;
	vertical-align:bottom;
	white-space:nowrap;
	color:windowtext;
	font-size:9.0pt;
	font-weight:400;
	font-style:normal;
	text-decoration:none;
	font-family:Geneva;
	border:none;
border-color: #00cc33;
border-width: 10px;
border: 1px solid black;
border-top: 1px solid black;
margin: 10px;
text-indent: 1em;
text-decoration: underline;
white-space:nowrap;
 }

<DIV STYLE="font-family:arial,helvetica; font-weight:bold; font-size: 16pt;
text-align: center;" ALIGN=CENTER>Magazine</DIV>
(https://[^"]*)">
\1" TARGET="_blank">

<STYLE type="text/css">
PRE, P  {
  font-family:Lucida Console, monaco, Monospace;
  font-size:9pt;}
</STYLE>

Terms:
XHTML DHTML HTML 5 CSS 3

Books:
- 10 Best Books on HTML & CSS
- Best CSS Books for Beginners
- CSS Book Recommendation - WebDeveloper.com forum
All the below got at least 4 stars at Amazon and recommendations in 2 or more lists.
- CSS: The Definitive Guide, by Eric A. Meyer 3rd eddition, 2007, was the only one which got 3 or more mentions in my search for CSS books, although I found some of the examples in others easier to follow.
- CSS: The Missing Manual David Sawyer McFarland, 2nd edition, 2009
- Pro CSS and HTML Design Patterns: Michael Bowers, 2007
Head First HTML with CSS & XHTML: Eric T Freeman, Elisabeth Freeman, 2006 - Amazon.com: Sams Teach Yourself HTML and CSS in 24 Hours, Julie C. Meloni, Michael Morrison 8th edition, 2010
Amazon.com: Stylin' with CSS: A Designer's Guide: Charles Wyke-Smith: Books, 2nd edition, 2008

Links:
CSS and Text | Text formatting, layout and presentation with stylesheets at YourHTMLSource.com
CSS Tutorial - List at tizaag.com