TinyMCE is a fantastic WYSIWYG editor available also for Drupal 7.x, but it does not support the iframe tag, or in a simple word if you try to embed a video, map, or an external web page you can see HTML code magically disappear. Fortunately the WYSIWYG IFrames module solves completely the problem and with a fix it is possible to maintain the allowfullscreen feature of the YouTube videos. Here is a few instructions and a screencast that simplify the whole process.
When I was planning my site, one of many problems I had, was insertion of mathematical equations in my articles, but I found very soon in MathJax a useful solution. Some time after I discovered another method: MathML, even a recommendation of the W3C math working group! Here there are some personal considerations on them: about usability and convenience in my Drupal experience.