We are so used to HTML now a days and most of us are comfortable with the HTML post editor on WordPress Edit Post or Add New Post screen. As we know the ‘Visual Editor’ is the default post editor for WordPress, but it sometimes adds non-standard code to the post’s HTML source or it also changes the post formatting that we have done thru HTML editor. So why not change this and set the HTML editor as the default post editor.
This is a very simple tip, just you need to paste only one line of code as embedded below in your active theme’s functions.php file
add_filter('wp_default_editor', create_function('', 'return "html";'));
Note that this will not disable the visual editor completely, it will just set the HTML editor as the default post editor, so that everytime you go to edit the post or add new post, you will be presented with the HTML editor.
[recommended]In case you need to disable the Visual editor completely: How To Disable The Visual Editor Without Using Plugin[/recommended]
Note:- You can find the Theme Functions file on your WordPress Admin Dashboard => Appearance => Editor => On the right hand side you can see the theme template files. Locate the Theme Functions file, add the line and save it.
In case you do not have access to Theme Editor on dashboard, you can always use the FTP client to get the file, edit it and then upload to the server.