CSS: Fix fontsize for mobiles

[GaelVaroquaux]

There used to be a CSS hack that increased the fonts size for high density display, to make it easier to read on mobile. This gave too large fonts on Macbooks with retina display. As a result, #375 removed this hack. But now the fonts are too small on mobile phone.

[rvyh]

Hi! yes, fonts are too small on mobile phone. But fonts are still huge on a macbook pro with retina display (1280 x 800, 13'). This is my whole screen:

A trick used by web developers is to add this on the head:

<meta name="viewport" content="width=device-width, initial-scale=1.0">

"Without a viewport meta tag, mobile devices render pages at typical desktop screen widths and then scale the pages down, making them difficult to read.

Setting the viewport meta tag lets you control the width and scaling of the viewport so that it's sized correctly on all devices."
source: https://web.dev/viewport/

After adding the code above you'd need to rezise fonts and tweak your media queries.

[stefanv]

Fonts are too big on Android (Galaxy S10 simulation in web console, and tested on physical device), FWIW.