You can ensure this by changing value for font-family attribute in following declaration in ~\CMSAdminControls\FCKeditor\skins\default\fck_editor.css file:

.SourceField
{
    padding: 5px;
    margin: 0px;
    font-family: Monospace;
}

You may need to clear browser's cache for change to take effect.

Please take a look to your CSS stylesheet and find following link CSS class definition:

.CMSTreeMenuTable a
{
white-space: nowrap;
}

It says to the tree menu not to break the lines. It should work if you remove it or comment it out.

The Header section of page template is not inherited from parent document's page template, if the document type used for this document is not set to "Behave as Page (menu item)".
You need to set the property to true:
Site manager -> Development -> Document types -> edit document type -> General -> Behaves as Page (menu item) type

Regrettably, neither styles from stylesheet specified in 'Site CSS stylesheet' nor 'Editor CSS stylesheet' are applied in toolbar of WYSIWYG editor. You will need to put styles also into ~\CMSAdminControls\FCKeditor\editor\css\fck_toolbarcombopreview.css file. You may need to clear browser's cache for these changes to take effect.

You can set the CSS class for search button (SearchButtonCssClass property of search box control).
If you set the class to ‘visibility:hidden’ it will be not displayed but the button will be in the rendered code so it will work.

The problem is that often the developers are using applications like MultipleIEs to have older versions of IE on the PC. They are testing it in this application and there is probably some bug in it when using IE6. When the site is visited in the default IE6 - it works fine, but it breaks in MultipleIEs e.g. on some post back action - the CSS styles are missing or corrupted. 

Yes, Kentico CMS produces valid XHTML code. Also the built-in WYSIWYG editor automatically cleans up the code so that it's XHTML compliant. You also have full control over the HTML (XHTML) code of the page layouts and document transformations, so you can always adjust the code so that it's XHTML compliant.

If your web site uses a dark body background, you may want to use a different background for the WYSIWYG editor. In such case, you need to specify the CSS styles like these:

.LTR, .RTL
{
  // set the dark background here
  background-color: black;
}

BODY
{
  // do not set any background here ...
}

The LTR/RTL style is used only for the web page background, while the BODY style is used for both the page and the WYSIWYG editor. Alternatively, you can create a separate CSS stylesheet and assign it to the editor in Site Manager -> Sites -> Site Properties -> General dialog.