Kompozer was removed from the repository as it is no longer being maintained by its maintainer in the upstream(Debian).
References:

• http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=690251


• http://packages.qa.debian.org/k/kompozer.html

As for your second question,
a suitable alternative for it would be Bluegriffon.

This is what it looks like:

To install it do the following steps:

1. Download the installer.


2. Right click on the Installer file. Pick the "Permissions" tab and
   check the Allow executing file as program checkbox. (For security
   reasons, most files aren't executable by default.)


3. Double-click on the installer file, and the GUI to install BlueGriffon will start.

About Bluegriffon, from its website:

BlueGriffon is a new WYSIWYG content editor for the World Wide Web.
Powered by Gecko, the rendering engine of Firefox, it's a modern and
robust solution to edit Web pages in conformance to the latest Web
Standards.

Partially reference: https://askubuntu.com/a/136849/11932

The latest bluegriffon can also be installed from the getdeb repository, by doing the following steps:

wget -q -O - http://archive.getdeb.net/getdeb-archive.key | sudo apt-key add -



sudo sh -c 'echo "deb http://archive.getdeb.net/ubuntu precise-getdeb apps" >> /etc/apt/sources.list.d/getdeb.list'



sudo apt-get update && sudo apt-get install bluegriffon

Courtesy: http://www.sourceslist.eu/software-tramite-repository/how-to-install-bluegriffon-editor-wysiwyg-on-ubuntu-12-04-from-ppa/