I want to know if i develop a visual web part, can i use html 5 tags in
.ASCX file without adding any link to source or making changes to master page ?
The doctype of the default master pages isn't for HTML5 so I doubt it, you would need to set up a new master page (even a duplicate of the built in ones) with the correct doctype to use HTML5.
You also have to then be wary of the CSS implementation as the master page normally forces IE10 compatibility but this could be resolved when you create your HTML5 master page.
I know of other issues - people picker not working when using newer browsers in SP2010 - not sure if its still an issue in 2013.
SharePoint 2013 out-of-the-box (both publishing or collaboration) has and "X-UA-Compatible = IE=10", see picture from my Office 365 site
Ideally it should be (e.g. IE=edge specifies to use the latest compatibility mode supported by your IE browser, which also by having IE=10 means support HTML5 Quirks mode - http://msdn.microsoft.com/en-us/library/hh869300(v=vs.85).aspx). To test it use any of the HTML5 specific tags, such as CANVAS, etc. and see if it simply works - put in a CEWP
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1" />
Have a look at this also http://msdn.microsoft.com/en-us/library/jj676915(v=vs.85).aspx
Do not use HTML5 tags if the DOCTYPE in your master page is XTML1.1.
Otherwise, you are just begging for trouble which can arrive in unexpected ways and places.