HTML, like most coding languages, has had improvements made over time that has removed older features, either due to them simply being problematic, or more often replaced with something better. If you continue to use these older features in your page, you may at the bare minimum not get the expected functionality in your page, or in the worst case, break some execution.
It is recommended to identify and remove any old or 'deprecated' tags from your code. This could be done manually if you have HTML or web design skills, or could be done by upgrading the template and library versions on your website.