Currently i use w3.css to a html page and use it in content editor webpart. The issue here is the CSS is affecting the whole page.How do i get the css to apply only to the content editor webpart.

i used the below link option 4 to add the content editor and the HTML file <http://sharepoint.hannahswain.eu/2014/10/10/adding-a-code-snippet-to-page-content-on-sharepoint-2013/>