CEWP(Content Editor WebPart) in SharePoint 2010 is used for Javascript Validation and HTML Static content. But in SharePOint 2013, when you place javascript and HTML tags, you would be taken back surprise that your HTML tags are removed automatically.
So ,what is the SharePoint answer for this behaviour?
The anwer is to use Script Editor Web Part for SharePoint 2013/ Office 365 which is a outofbox webpart that comes with SharePoint 2013. Its not available in SharePoint 2010 though.
So your next question is what is the usage of CEWP in SharePoint 2013? Microsoft suggest to use it for static content not even HTML tags.
To understand more about Script editor and Embed coded webpart, Please read my
blog (http://www.c-sharpcorner.com/Blogs/14603/script-editor-webpart-vs-embed-code-in-sharepoint-2013.aspx)
Happy SharePointing :-)