Where can In find the html code of my new custom master page

I created a new custom master page.In the developer tools of IE Iam able to see the code with many divs with different classes,but when I open the custom_master_page.html ,I cant find any of those code.If I want to introduce a new div into the master page layout Iam unable to do it because couldnt identify where exactly the code lies.The html page of the master page did not contain any of the code that Iam viewing in the developer tools on the browser of the same page.