I will put up app's for iOS and Android later.
Even the little Kobo "Arc" reader tablet lets you sideload CJK Unicode fonts ( sorry, I am only working with Unicode fonts, but I will do browser applets and mobile app's for Shift-JIS font testing if you request )
One applet shows all of your fonts which are capable of displaying the kanji character for 'zen'.
For web fonts, it is important to know the size of the, say, TTF, file ... I have not worked that out yet ( it may have to be pulled from font web source pages )
Curl is a Unicode web content language from MIT that dates back to the beginning of WWW's w3c DARPA grant ... it remains a great alternative to HTML + CSS + JAvaScript ... and has both doc's support pages in Japanese !
The doc pages are themselves Curl and run LIVE code that you can edit as you explore the documented features ( and restore to the original with one click ! ) The support pages have "live" users but response delay may depend on your time zone
I have some posts on these applets over at blogger as http://lcurlr.blogspot.ca/ and http://lcurlr.blogspot.ca/2013/08/your-fonts-japanese.html as well as my usual http://kanjirecog.blogspot.ca/ and http://kanjirecog.blogspot.com/ as well as my twitter, facebook, linkedin and google+ pages ...
I will be adding font choice for the user to most of my Curl applets and app's now that I see what a HUGE difference this can make depending on the device I am using and the app that I am viewing/using. Watch for a special use of BOLD Gothic kanji fonts, coming soon !
If you DO use only HTML with CSS, see if your browser supports RANGE and try getting katakana into Gothic font, hiragana into a Light Mincho font and Kanji into a font where complex kanji look best on your device at your preferred resolution, brightness and contrast. And if you wear glasses/contacts, take a moment to check your distance, if not your specs prescription ! Remember : blinking helps moisten eyes and reduce strain BUT web pages may lead to reduced blinking, so ... BLINK a few, use a safe eye drop and learn to massage eyelids gently to keep away that redness !
