I am creating my website on github page, I am not using jekyll or any of its kind, it's just pure javascript and html css. Why sometime it gives page build failure? what it is building then?
By default in a github repository, the gh-pages branch is build with Jekyll.
If you have underscored folders (eg: _myfolder) Jekyll tries to build. If they are not real Jekyll folders, this breaks the build.
If you don't want to process you html/js code, add a .nojekyll file at the root of your code. Github will then not try to build, it will just serve your pages as is.
More info in Github pages doc