Question

I want to use css3 features like

box-shadow

so for that

1. Do i need to include some special tag for css3 at top of page
2. which browsers currently support it
3. what will happen if use many css3 features and people view in old brosers.cause any error or so
4. Should i really use it or not

Answer 1

Do i need to include some special tag for css3 at top of page

No

which browsers currently support it

Most browsers support some CSS 3. No browser supports all CSS 3. When can I use tracks when support for various features was added.

what will happen if use many css3 features and people view in old brosers.cause any error or so

Unless the browser is exceptionally buggy, the error handling rules of CSS will apply and the unrecognised thing will be ignored.

This is only a problem if another style (which does apply) will render content unreadable unless the unsupported style is also applied. This probably can sometimes be worked around by applying the same property twice. First with widely supported values, then with less widely supported ones: e.g.:

color: white;
background: url(blue_0.5_pixel.png);
background: rgba(0%, 0%, 100%, 0.5);

Should i really use it or not

That needs to be determined on a case-by-case basis.