gouri, how about this? Stop worrying about what helps ranking and what doesn't. If you do things naturally without over thinking things, you'll probably be just fine.
You may want to read through these documents carefully. Everything you need to know about SEO is right here...
HTML 4.01 Specification
Index of Elements
Index of Attributes
After reading and assimilating the above information, you will be leaps and bounds ahead of many others. Everything you need to know is written above, you just need to read and understand it.
There are no conflicting opinions in the specifications above. Stop reading all that other crap and read the authorities on the subject. Use the Search Engine Webmaster Guidelines in addition to the above and you'll be just fine.