Is there a front end WYSIWYG website editor that can edit a webpage without having to install a CMS?
FTP-based content management systems such as the following offer hosted solutions to managing simple content via a WYSIWYG-style interface:
You provide them with the FTP details for your existing site, and they allow you to set editable regions by adding class names to your HTML, which lets clients edit sites directly via a WYSIWYG without having to install software, without having to learn HTML, and without having to make major changes to their existing site.
They can be a little more restrictive in terms of feature set and security than self-hosted content management systems, but they're useful for simple sites and low budgets.
There are also self-hosted equivalents to the above, such as Unify, in case they're of interest.