The general summary is that we can reasonably expect all browsers, both modern and very old, to have different space character widths for serif and monospace fonts. We can use this knowledge to remove the need to specify sampleText options in most cases.
Mac Modern Browsers ------------------- Chrome 52.0.2743.116 (Mac) 249 - serif 277 - sans-serif 599 - monospace 277 - cursive 226 - fantasy Firefox 47.0 (Mac) 250 - serif 278 - sans-serif 600 - monospace 278 - cursive 227 - fantasy Opera 39.0.2256.48 (Mac) 250 - serif 278 - sans-serif 600 - monospace 278 - cursive 227 - fantasy Safari 9.1.2 (11601.7.7) (Mac) 250 - serif 278 - sans-serif 600 - monospace 278 - cursive 227 - fantasy Windows Modern Browsers ----------------------- Chrome Version 52.0.2743.116 m (Windows) 249 - serif 277 - sans-serif 549 - monospace 298 - cursive 176 - fantasy Firefox 48.0.2 (Window) 250 - serif 278 - sans-serif 600 - monospace 299 - cursive 278 - fantasy Opera 39.0 (Windows) 250 - serif 278 - sans-serif 549 - monospace 299 - cursive 177 - fantasy Edge 25.10586.0.0 250 - serif 278 - sans-serif 600 - monospace 299 - cursive 174 - fantasy Internet Explorer 11.494.10586.0 250 - serif 278 - sans-serif 600 - monospace 299 - cursive 174 - fantasy Internet Explorer 11.0.9600.17843 250 - serif 278 - sans-serif 600 - monospace 207 - cursive 174 - fantasy Old Versions ------------ Internet Explorer 10.0.9200.17457 333 - serif 278 - sans-serif 600 - monospace 207 - cursive 174 - fantasy Internet Explorer 9.0.8112.16421 333 - serif 278 - sans-serif 600 - monospace 207 - cursive 174 - fantasy Internet Explorer 8 250 - serif 278 - sans-serif 599 - monospace 299 - cursive 250 - fantasy (took a timeout to see the width of a space-only div???) Internet Explorer 7 250 - serif 278 - sans-serif 599 - monospace 299 - cursive 250 - fantasy (took a timeout to see the width of a space-only div???) Internet Explorer 6 250 - serif 278 - sans-serif 599 - monospace 299 - cursive 250 - fantasy (took a timeout to see the width of a space-only div???) Chromium 4.0.207.0 (25573) (Mac) 250 - serif 278 - sans-serif 599 - monospace 299 - cursive 176 - fantasy Safari 5.1.7 (7534.57.2) (Windows) 250 - serif 278 - sans-serif 599 - monospace 299 - cursive 299 - fantasy