I know that a lot of us are familiar with setting the font size on the body element in our CSS to 62.5%. This means that 1em will equal 10px and helps for keeping things pixel perfect but also allows for scaling of fonts.
So wouldn't that mean that setting it to 6.25% would equate to 1em = 1px? Seems like an even simpler conversion rather than having to mess with decimals...
Arguably, but then you lose control over your scale. Don't forget that headings will typically inherit those same sizes in proportion to their rank (i.e. <h1> will be largest, <h2> slightly smaller). If you want to decrease those elements, you will need to use em values with a lot of decimal placeholders. Imagine <h4> font-size: 0.005em.
Or worse, if you want fonts to be scaled larger, you could potentially be looking at font-size: 40em or something ridiculous.
In short, 1em = 10px is much more practical for the scaled values of fonts. While a 1:1 scale might make sense on paper, it doesn't lend itself that well to sensible and maintainable CSS.
The conversion may be simpler, but an em wouldn't mean what it is supposed to mean.
1em is supposed to be equal to the width if a capitalized "M" in a given font. If the width of the letter M is 1 pixel, your font is going to be unreadable.
First of all, do not assume that 1 em will equal 10 pixels. An em unit is in direct correlation to the typography being used. If someone has a font size of 16 pixels, then 62.5% is indeed 10 pixels (16 * 0.625 = 10) but this will obviously change when someone has modified their default font size.
Secondly, this is the first I've ever heard of using 62.5% for the base body font-size. I always use a font-size of 76% as based on Sane CSS Typography by Owen Briggs.
Lastly, to answer your question, yes you could use a font-size of 6.25% and then use 12em instead of 1.2em, for example. However, I would highly discourage this methodology. In the world of typograhy, one em is intended to be the width of the capital letter 'M'. This method completely violates that common practice and will seriously confuse anyone that may maintain your CSS in the future.
Thanks guys! I'm quite aware of the em and it's history (design degree), but I'm sure others may find it helpful :)
As far as the 1em = 1px, I don't see how this is undesirable. The em is square, regardless of your units (be it points or pixels) and nobody would set their type at 1px (just like nobody would set printed type at 1pt). Furthermore, even your article concedes that in most digital typefaces, the capital "M" is usually smaller than 1em, and that the em is merely a reflection of the point size (48pt type would render a 48pt by 48pt square for the em, 12pt type would yield 12x12, etc.)
Besides, the reason people would do this would be more for setting dimensions of other elements on the page so that everything scales nicely when the user adjusts their font size. Sure, there will always be the rare few who set their default to something other than 16px, but well worth the price to pay for a pixel perfect layout that scales nicely.
The whole "62.5%=10px" thing is fundamentally broken anyway - 62.5% may or may not be 10px depending on the browser, the user's settings, and, especially, the minimum font size setting. So you can't just design in pixels and then "convert" to ems on the assumption that 62.5%=10px, because your design will break all the time. If you want a pixel-perfect design, you have to use pixels as the unit. If you want a flexible design, you need to think about the appropriate units for different elements of the web site - ems for elements which should scale relevant to text size, percentages for elements that should scale relative to window size, and pixels for elements (like images) that shouldn't scale at all.
Anyone who includes font-size: 62.5% in their CSS fundamentally doesn't understand how to design for the web.
You might find this useful as well. http://pixel2em.kleptomac.com This provides an online pixel to em converter and you can also do a complete CSS file conversion.
An updated version is available at http://pixelconverter.kleptomac.com Its an online unit converter for converting pixels, point, em, percentages. This supports conversion from/to any of these units.