If, like me, you find it very annoying that Umbraco seems to stop you from entering whatever markup you want to put in the editor, then this post should help you.
There is a setting in the tinyMceConfig.config file which should be in the /config folder.
The setting is called validElements and usually looks like this:
<validElements>
<![CDATA[+a[id|style|rel|rev|charset|hreflang|dir|lang|tabindex|accesskey|type|name|href|target|title|class|onfocus|onblur|onclick|ondblclick|onmousedown|onmouseup|onmouseover|onmousemove|onmouseout|onkeypress|onkeydown|onkeyup],-strong/-b[class|style],-em/-i[class|style],-strike[class|style],-u[class|style],#p[id|style|dir|class|align],-ol[class|reversed|start|style|type],-ul[class|style],-li[class|style],br[class],img[id|dir|lang|longdesc|usemap|style|class|src|onmouseover|onmouseout|border|alt=|title|hspace|vspace|width|height|align|umbracoorgwidth|umbracoorgheight|onresize|onresizestart|onresizeend|rel],-sub[style|class],-sup[style|class],-blockquote[dir|style|class],-table[border=0|cellspacing|cellpadding|width|height|class|align|summary|style|dir|id|lang|bgcolor|background|bordercolor],-tr[id|lang|dir|class|rowspan|width|height|align|valign|style|bgcolor|background|bordercolor],tbody[id|class],thead[id|class],tfoot[id|class],#td[id|lang|dir|class|colspan|rowspan|width|height|align|valign|style|bgcolor|background|bordercolor|scope],-th[id|lang|dir|class|colspan|rowspan|width|height|align|valign|style|scope],code[class],caption[id|lang|dir|class|style],-div[id|dir|class|align|style],-span[class|align|style],-pre[class|align|style],address[class|align|style],-h1[id|dir|class|align|style],-h2[id|dir|class|align|style],-h3[id|dir|class|align|style],-h4[id|dir|class|align|style],-h5[id|dir|class|align|style],-h6[id|style|dir|class|align|style],hr[class|style],dd[id|class|title|style|dir|lang],dl[id|class|title|style|dir|lang],dt[id|class|title|style|dir|lang],object[class|id|width|height|codebase|*],param[name|value|_value|class],embed[type|width|height|src|class|*],map[name|class],area[shape|coords|href|alt|target|class],bdo[class],button[class],iframe[*]]]>
</validElements>
This contains all of the rules about what elements and attributes are valid in the markup for the Rich Text Editor.
You can add extra rules in to allow the elements and attributes that you want, or you can turn it off completely and allow all elements by changing it to say the following:
<validElements>
<![CDATA[*[*]]]>
</validElements>
See the official documentation here