CSS – Initial HTML Margin Issue On WordPress Site

That looks like the margin added by the WP admin bar when you are logged into the site. WP adds an inline style tag which sets the margin on the html element with an !important flag. Since it typically comes after any of your own styles it overrides them even if you also use !important. Since it only displays when you are logged in it shouldn’t matter much, but if you really want to remove the admin bar you can change that in the settings Users->Your Profile then uncheck “Show Toolbar when viewing site”. Or add the following to your functions file.

add_filter(‘show_admin_bar’, ‘__return_false’);