Ckeditor changes  tags to  tag
For FILTER HTML go to:

/admin/config/content/formats/filtered_html

add &lt;button&gt; to the limit allowed HTML Tags list.

If you use FULL HTML the button should work.
If not try
go to:
/admin/config/content/ckeditor/edit/Advanced (for Filter HTML)
/admin/config/content/ckeditor/edit/Full (for Full HTML)
Then under Advanced Content Filter, select disabled.