Is there any site where you can actually practice mathjax and where it shows how it looks like? Problem is that I can't post because the site keeps on saying" does not meet quality standards" problem is I can't see how it looks like unless it gets posted in the first place. Any help is welcome
When you click on "Ask a question" on the main site, preview should be displayed when you start typing the question. This is not done immediately, but when you stop typing for a second, math should be rendered; as shown in screenshot below.
If you want, you can then copy the text and save it to the file on your local computer. (Or you can work the other way round; once you have mastered TeX syntax at least a little and you have a text editor which you are comfortable with, you will probably prefer to prepare the text in your favorite editor, where you can use all shortcuts and UI you are used to, and then copy it to the browser and do minor correction there.)
This seems to be the easiest way to practice mathjax. You can also use some of the sandboxes we have here.
Another possibility could be to prepare a html-file on your local computer and view it in your browser. Unless you want install a local copy of MathJax, you need to have access to internet and you need include the necessary scripts in the page header. See MathJax website for more information. The rendered output might look different from what you have seen on this site, since there are many things you can configure.
Another viable possibility could be simply to prepare a html-file and with the text you need; or type in any edit field. If you install mathjax bookmarklet, you can then render you math with one click to preview it.
There are probably many other sites which use MathJax and perhaps some of themcould be used for testing, if you don't like the above options for some reasons. Many such websites are mentione here.