How to set min-font-size in CSS

In CSS3 there is a simple but brilliant hack for that:

font-size:calc(12px + 1.5vw);

This is because the static part of calc() defines the minimum. Even though the dynamic part might shrink to something near 0.

As of mid-December 2019, the CSS4 min/max-function is exactly what you want:
(tread with care, this is very new, older browsers (aka IE & msEdge) don't support it just yet)
(supported as of Chromium 79 & Firefox v75)

https://developer.mozilla.org/en-US/docs/Web/CSS/min
https://developer.mozilla.org/en-US/docs/Web/CSS/max

Example:

blockquote {
    font-size: max(1em, 12px);
}

That way the font-size will be 1em (if 1em > 12px), but at least 12px.

Unfortunatly this awesome CSS3 feature isn't supported by any browsers yet, but I hope this will change soon!

Edit:

This used to be part of CSS3, but was then re-scheduled for CSS4.
As per December 11th 2019, support arrived in Chrome/Chromium 79 (including on Android, and in Android WebView), and as such also in Microsoft Chredge aka Anaheim including Opera 66 and Safari 11.1 (incl. iOS)