Question

Use html tag to display hydrogen symbol.

Answer 1

I am not sure what you are looking for here.  The symbol for hydrogen is just uppercase "H".  So any way you display an H would work, no?

For example: <h1>H</h1>  or <span>H</span>  etc.

Answer 2

Their is element called sub .

<sub> :- The <sub> element is used to contain characters that should be subscript. It is commonly used with foot notes or chemical formulas such as H20.

Example:- <p>Formula of water is H ₂ O. </p>