This module makes adding additional settings to the TinyMCE init call simple.
After installing the module, copy the following file
js/pulsestorm_tinymceconfig.dist.js
to
js/pulsestorm_tinymceconfig.js
Add your additional settings at the following location in the above file
var settings = this.originalGetSettings(mode);
//add any extra settings you'd like below
//makes "placeholder" a valid element for inputs
settings.extended_valid_elements = 'input[placeholder|accept|alt|checked|disabled|maxlength|name|readonly|size|src|type|value]';
return settings;
Original Article: http://alanstorm.com/magento_html5_tinymce