This behavior is due to the default implementation of the RadEditor our content blocks use. More specifically it is due the editor's filters towards the HTML inside. You can actually reproduce the behavior on our online sample
. Paste your HTML, switch to design and then back to HTML and you will see the same behavior present.
Sitefinity applies all RadEditor filters from its configuration. In order to remove the filters you need to go to Administration->Settings->Advanced->Appearance and set a value of None to both "RadEditor filters for Content block widget" and "Rad Editor's content filters". Remember to restart the application for the changes to take effect.
Ivan D. Dimitrov
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