CSS max-width versus width properties for text columns

It's easier to read on a web page if the columns aren't too wide. This presentation Ideal line length for content suggests setting the values of the min-width and max-width CSS properties around 30 em. Columns of 30 em sounds very narrow for a PC-based browser, but I expect it makes sense for browsers on handheld devices.

Using the Stylish Firefox add-in, here's my Narrow Paragraph rule for sites that I read:

@-moz-document domain(<domain 1>)
  , domain(<domain 2>) {
  body {
    min-width : 25em;
    max-width : 30em;
  }
}

Earlier, I used the width CSS property but when the browser window is made narrower than the width's value, the width of the text column doesn't change and you have to scroll left and right to read. Setting both min-width and max-width gives the designer and reader some flexibility in layout and viewing, respectively.