Redefine Constant in php

runkit_constant_redefine — Redefine an already defined constant

Beginner questionfor html

Every .html page we create requires tags to start and end html file an recognize it as a html file.
But even when I don't give any HTML tags and simply write a text in .html file, the file gets opened in the browser with the text I have written in the .html file without using any of the HTML tags.
How is the text getting displayed in the browser without using any HTML tags?
Does the browser automatically add HTML tags behind the scenes??
When I viewed the page source in that also it shows simply the text not the HTML tags...