Gutenberg editor uses special CSS to better manage blocks. If you are using the new WP editor, it is most possibly you will have the following code in your theme:

<link rel='stylesheet' id='wp-block-library-css'  href='' type='text/css' media='all' />

If you are not using Gutenberg blocks in anyway, you can use the following snippet to remove this extra stylesheet from your theme;

function wpassist_remove_block_library_css(){
    wp_dequeue_style( 'wp-block-library' );
add_action( 'wp_enqueue_scripts', 'wpassist_remove_block_library_css' );