What are the steps we must take to be professional website designers and what standard that we must know ?

I can suggest few things not that I may be completely correct , It's very difficult to master every technology , I believe each will have it's own learning curve .

But there are many good built in tools , also external tools that will give you a second eye of how your  code is written . 

Few tips aroiund coding ,

For JS -> be sure of how you're manipulating the DOM , appending and wrapping divs are expensive operations and as the iterations increase , the DOM layer takes a toll and coudl hang the site .

For CSS -> my first priority in modern world would be taking care of cross devices and resizing , making sure that my site changes accorsing to the browser . Ideally no other layer should handle the css (i.e JS or ASP.NET should be purely responsible for manipulating DOM and generating the DOM) , CSS shoudl take complete care of how your site looks.

VS has analysers , profilers . sIEve gives you good analysis of the DOM. Another thing that myseld didn't learn but realised it's importance is SOLID design principles. easy to learn , extremely difficult to master.