By default WordPress limits what HTML tags are allowed in post content. If tags that are not whitelisted are found in the content they will be stripped. It is possible add additional tags to the allowed list. https://ben.lobaugh.net/blog/201762/add-html-tags-to-the-allowed-tags-list-in-wordpress
This is very nice that you can control which HTML tags can be used in Wordpress. It gives you great control over the look of your sites, and that's a good thing.
The site quoted above explains how to allow certain tags in your Wordpress installation, but not just in your installation overall, but also in the Wordpress editor.
So if you want to add <iframe> or <blink> to your allowed HTML tags you could.