Hi everybody

I am building a little options menu with text. Text buttons are PNGs images displayed as texture on mesh.
I find that my text buttons are not realy sharp displayed, and when I scale the Blend4web viewer window the text is not readable anymore:

in 933x774 is ok

in 558x481 is not ok anymore

The text textures are PNGs with transparency, as you can see in my blend file:

How can I optimise the text textures to make them sharper and good scalable, that they can be better readen ?

Should I use an other texture image format (JPG, GIF…?) ?
Should I deactive the transparency and use text on white background ?
Are nodes materials/textures better displayed than basic materials/textures (I use basic materials/textures) ?

Thank you for your help !