Having trouble with HTML in posts

Are you using the TEXT editor and not switching editors?

Can you post a link to your site?

There are two “editors” – at the top of the editor box there are two tabs – one “Visual” the other “Text” – try using the TEXT editor for adding HTML:

http://codex.ww.wp.xz.cn/Writing_Posts#Visual_Versus_Text_Editor

Note that switching between editors will mess up your code – so stick with the HTML editor if you are adding code.

If that does not work, try deactivating your plugins and switching to the twentythirteen theme to see if the problem persists.