Many times, we may need to put additional comments in our style sheet blocks. We can simply put our comments inside.
/*.....this is a comment in style sheet.....*/

If we want use the same style to multiple pages, we should define styles as a separate style file. These separate style files are known as Sitewide Style sheets or External Style Sheets.

The < link > tag is used to add CSS file with HTML in head section. While 'using < link > tag, the following attributes' are also included along with standard values.
reI = "style sheet"
type = "text/css"
The href attribute is used to link the css file.

< style >
h3
{color.blue;
font-family: "Copperplate Gothic Bold";
font-size: 14pt;}
< /style >

General format of <Link> tag
<Link rel = "Stylesheet" type = "text/css" href = CSS_ File_Name_with_Extension>

Linking CSS with HTML:
The < link > tag is used to add CSSfile with HTML in head section. While using < link > tag, the following attributes are also included along with standard values.
rel = "stylesheet"
type = "text/css"
The href attribute is used to link the .css file.
General format of < Link > tag
< Link rel = "stylesheet" type = "text/css" href = CSS_File_Name_with_Extension >
HTML Code for Formatting HTML element through CSS:
Mystyle.css
H1
{
font-family: "Comic Sans MS";
Font-weight: Bold;
border:2px solid blue;·
}
P {
font-style : Italic;
color :MediumSeaGreen;
}
--- CSS_Test.htm --- 
< html >
< head >
< title > Demonstration of using CSS < /title >
< link rel="stylesheet" type="text/css"
href= "mystyle.css" >
< /head >
< body >
< H1 > Welcome to CSS < /H1 >
< p >
CSS was invented by HakonWium Lie on October 10, 1994 and maintained through a group of people within the W3C called the C5S Working Group. The CSSWorking Group creates documents called specifications. When a specification has been discussed and officially ratified by W3C members, it becomes a recommendation.
< /p >
< body >
< html >

H1
{
Font-family
}
Font-family : "Arial";
        Font-style : Italic;
        Font-size : 24px;
        }

{
color: red; Text-align : center;
}

CSS style declaration consists of two major parts; Selector and Declaration. The Selector refers an HTML tag in which you want to apply styles. The Declaration is a block of code that contains style definition. It should be surrounded by curly braces. Any number of properties for each selector can be included and they must be separated by semicolons. The property name and its value should be separated by a colon. Each declaration should be terminated by a semicolon (;).

(i) Maintainability : CSS are also defined and stored as separate files. So, the style and appearance of a web page can be dynamically changed and maintained with less effort. 
(ii) Reusability : The styles defined in CSS can be reused in multiple HTML pages.
(iii) Easy to understand : The tags in web pages are well organized with style specifications and therefor it is easy to understand.

Formatting	Properties	Values	Example
Background Image	Background-Image	URL ("image name with extension'')	Body { background-image: url ("Flower.gif''); }
	Background-repeat	norepeat	Body { background-image: url ("Flower.gif''); background-repeat: norepeat; }
	Background-position	Direction	Body { background-image: url ("Flower.gif''); background-repeat: norepeat; background-position: right top; }
Paragraph Margin	Margin-top Margin-bottom Margin-left Margin-right	Margin size in pixels	P { Margin-top: 50px; Margin-left : 50px; }

Formatting	Properties	Values	Example
Text Colour	Color	Predefined _Color_Name	P { color:pink; } H1 { color:MediumSeaGreen; }
Text Alignment	Text-align	Centre / Left / Right / Justify	P { Text-align: centre; }
Font type	Font-family	Font_name	P {  Font-family: "Times New Roman" : }
Font Style	Font_Style	Normal/Italic	P { Font-family: "Times New Roman" Font-style: Italic; }
Font Style (Bold)	Font-weight	Normal/Bold	P { Font-family: "Times New Roman"; Font-style: Italic; Font-weight: bold; }
Font size	Font_size	Size in pixels	P { Font-family: "Times New Roman" Font-style: Italic; Font-size: I4px; }
Background Colour	Background Colour	Predefined_Color_Name	Body { background-color: violet; }
Border Colour	Border	Borer thickness < space > border style < space > Predefined_borderColor_Name	H2 { border: 2px solid red;}
HTML supports nearly 140 color names
Background Image	Background-image	URL("image name with extension")	Body { background-image: url ("Flower.gif"); }
	Background-repeat	norepeat	Body { background-image: url ("Flower.gif"); background-repeat: norepeat; }
	Background-position	Direction	Body { background-image; url ("Flower.gif"} ; background-repeat: norepeat; background-position: right top; }
Paragraph Margin	Margin-top Margin-bottom Margin-left Margin-right	Margin size in pixels	P { Margin-top: 50px; Margin-left: 50px; }

< html >
< head > < title > CSS DOCUMENT < /title >
< style >
P
{
font-family : "Cooper Black"; Font-style: Bold: Font-weight: Italics; Color: Blue;
}
< /style >
< body >
< p >
The State Institute of Education (SIE) was established in 1965 to provide for systematic study of problems relating to School Education under the administration of Directorate of School Education.
< /p >
< /body >
< /html >