You may find that the WordPress editor is stripping HTML tags when moving from the visual editor to the text editor.
HTML tags like <br> , <p> ja <span> tags are being removed, when clicking on the text editor whilst in the visual editor.
The removal of the tags can make break the intended layout and can even make some content inaccurate or incorrect.
The fastest way to stop the stripping of the HTML Tags and still allowing to switch between the visual and text editors is to install the Preserved HTML Editor Markup WordPress plugin. Once the plugin is installed and activated, it requires no configuration and WordPress will no longer strip the HTML tags when moving from the visual to text editor.