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Deprecated Features
When you enter HTML code in our WYSIWYG editor, we scrub your HTML code pretty hard.
There are several reasons for this.
Security. Allowing unlimited HTML brings a bunch of security risks, known as "cross-site scripting" attacks, or XSS for short. By restricting the HTML, we eliminate that.
Content Spam. Spammers have long used links as a way to game Google. We prevent that from working.
Because when you copy-paste content from other programs into the wysiwyg editor, it'll usually come with a bunch of markup that you don't want. MS Word is especially awful in this regard, but other software does it too. Fonts, colours, background colours, and other things that you don't want, in addition to just awful and bloated markup that introduces weird problems. By restricting HTML, we manage to remove most of the unwanted markup.
You as an admin are allowed more freedom than your end customers.
I hope this explains the reasoning.