From MODx Wiki

Note: This has been fixed in the latest SVN trunk of MODx 0.9.5. All users using prior versions will need this update.

Note 2: This only works in TinyMCE versions 2.0.6.1 and newer.

Contents 1 Fixing Source Formatting 1.1 Turn On Apply Source Formatting 1.2 Turn Off Removing Linebreaks 1.3 You're Done!

Fixing Source Formatting

To prevent TinyMCE from stripping your newlines and uglyifying your HTML, in:

/assets/plugins/tinymce/jscripts/tiny_mce/tiny_mce.js

Turn On Apply Source Formatting

this._def("apply_source_formatting", false);
TO
this._def("apply_source_formatting", true);

Turn Off Removing Linebreaks

this._def("remove_linebreaks", true);
TO
this._def("remove_linebreaks", false);

You're Done!

And viola, nicely readable source.

Alternatively you can simply override these parameters, when initiating tinyMCE:

tinyMCE.init({
    mode : "textareas",
    theme : "advanced",
    apply_source_formatting : true,
    remove_linebreaks: false
});