How to write equations in html?

Try mathjax http://www.mathjax.org/ . I personally find it very good.

For complex equations, MathJax is the current practical solution. MathML is a more structural approach in principle, but browser support is rather limited and often of questionable quality.

However, complexity is relative. To some people, E = mc² or ∂/∂t + v ⋅ ∇ might be complex, and such constructs can be written fairly well using just HTML with some help from CSS; see my page Math in HTML (and CSS).