Every publisher, except a few, know HTML very well. Almost all of them use the HTML Editor to publish new content from the WordPress dashboard. But WordPress, to ease the content creation process, also provides the visual editor for people who are not good at HTML.
Using the text editor (HTML) with the visual editor enabled is not a problem when creating new posts, but it seems to be one when you try to edit an older post that was created using the visual editor. You will have a hard time adding all the tags and attributes because WordPress editor doesn’t displays them in the text editor. Also, switching between Visual and Text editor will remove many tags that you may have added to your content. The easiest thing to prevent all these problems is to disable the Visual editor altogether.
Disabling The Visual Editor in WordPress
Adding the below code to your functions.php file will disable the visual editor completely. Read this article to know how to edit functions.php file.
add_filter('user_can_richedit' , create_function('' , 'return false;') , 50);
NOTE Make sure to paste the code before closing the PHP ( ?> ) tag.