The app provides:
- edit up to 3 text files (assumed for html/css/js you need to create them first even empty)
- simple switching between them (use buttons "F1"/"F2"/"F3")
- save files locations and edit panel font size configurations (use button "Presets")
- review created web in the viewer next to edit area (just tap "Save&Refresh" button)
- access to local web-server by IP (check "Use IP" and enter server IP)
- open your web in external browser (IP mode only use long click on the "Save&Refresh" button)
- adjustable viewport width for common dimensions (use "Width" button).

Ad free.

Back button functionality is not provided in the web viewer. Please be careful to use external links.

The app "Server for PHP" https://play.google.com/store/apps/details?id=com.esminis.server.php was tested as device local web-server.

Warning! The app can't provide the following:
- code syntax highlighting
- line numbers
- ftp access
- autosave
- lots of abilities which are provided by serious apps.

The app was created in MIT App Inventor (http://appinventor.mit.edu).