Html – list of mobile browsers which I must support and test for

You can use this trick for converting fonts from px to em in your CSS:

body {
  font-size: 62.5%; /* resets the page font size */
}

Then specify your font sizes like this:

p {
  font-size: 0.8em; /* equals 8px */
  font-size: 1.0em; /* equals 10px */
  font-size: 1.6em; /* equals 16px */
  font-size: 2.0em; /* equals 20px */
}

And so on. Then you can convert px to em for your layout at PXtoEm.com.

Update: So apparently, WebKit is a HTML/CSS web browser rendering engine for Safari/Chrome. Are there such engines for IE/Opera/Firefox and what are the differences, pros and cons of using one over the other? Can I use WebKit features in Firefox for example?

Every browser is backed by a rendering engine to draw the HTML/CSS web page.

• IE → Trident (discontinued)
• Edge → EdgeHTML (clean-up fork of Trident) (Edge switched to Blink in 2019)
• Firefox → Gecko
• Opera → Presto (no longer uses Presto since Feb 2013, consider Opera = Chrome, therefore Blink nowadays)
• Safari → WebKit
• Chrome → Blink (a fork of Webkit).

See Comparison of web browser engines for a list of comparisons in different areas.

The ultimate question... is WebKit supported by IE?

Not natively.