There are a few reasons that can cause this problem. 

Not allowed HTML markup

WordPress can remove extra formatting when you switch between visual and text tabs. This case can be simply tested in default WordPress editor as shown on the example below

Your text can look different on the front end if that is the case . List of allowed tags can be found here.

Unnecessary HTML markup

Some of the 3rd party resources or text editors such as Word can add have HTML tags in the text, these tags are not visible however when you copy text and paste to WordPress editor all tags are also copied. You can see these tags in text tab of the WordPress editor. 

To prevent it you can use online tools such as this one to remove all HTML tags from the text.

Broken HTML markup

Text on front end may look different if you have broken HTML tag in text, it also may affect all other text on the page. Example of the correct markup

<h1><a href="#">Link Text</a></h1>

If one tag is missing it will affect all other HTML code on the page, here is the example

<h1><a href="#">Link Text</a></h1

You can use this tool to check if your code is valid and fix the errors.

Issues related to CSS code

If you see that font size, color, alignment or similar properties do not work on front end this might be related to incorrect CSS code. In this case you will need to make sure that CSS code is valid and doesn't have any mistakes. To check CSS code please follow this article.

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