How can we make web page (having css) which can view same in all web browser? eg:css property - backround-size (in ie6) not supported

I will recommend you to use a css framework. That will make your work very easy and cross browser compatibility is ensured by the framework itself. Just as an example bootstrap...

http://getbootstrap.com/

Minimize your work and have a life :)

First, webpage not nessecary to look the same in all browsers, specially when we are talking about old browsers (IE8 and below).

http://dowebsitesneedtolookexactlythesameineverybrowser.com/

For Modern browsers to easier to page look the same (Firefox, Chrome, Safari, etc..).

1. You will need to use reset css in first place to reset any browser default styling that to any element in the page (body, table, p, ul, li, etc.....).
2. You need to make sure that you are writing standard css code that common and readable in all browsers.
3. For CSS3, you will need to add css3 properity related to these browser, ex:

.box_with_rounder_corners {            -moz-border-radius:6px;            -webkit-border-radius:6px;            border-radius:6px;}

Link below help you to generate the css3 code you need to support browsers:

CSS3 Generator

For old IE browsers (IE8 and below):

1. You will need to add css workarounds in order to fix any issue in page layout, you eaither use css hacks for these browsers, or you can include css files seperate to each browsers inside head tag
2. to support CSS3 you have one of two options:

a- using css3 workarounds (most probably the filter property), an examples below.

CSS3 Solutions for Internet Explorer

b- using a js plugin to support css3, like Modernizr, IE-CSS3, CSS3Pie, etc....