Question

Given that Twitter Bootstrap is designed to be responsive / device-friendly, why doesn’t it use relative font sizes?

Answer 1

Well it seems that they are hiding behind the browser zoom excuse. Really sad to see such a heavily used and influential framework completely ignore accessibility issues and a fundamental cornerstone of responsive design. They are in a position of great responsibility and unfortunately seem to have no intention of acting accordingly.

[Update] So today Mark Otto replied on the thread I referenced above. Predictably there is no mention of accessibility and use of the phrase ‘pixel-perfect’:

Okay, so here’s a bit of a background on the decisions of yesteryear
and plans for moving forward.

Pixels provide absolute control and consistent rendering across every
browser.

Designers still mostly think and operate in pixels.

Browsers
scale up entire pages these days, so it’s not an issue with type
scaling or anything.

Nesting ems historically has been a pain and can require extra math for figure computed/intended pixel values.

Mixing units of measurements is ugly and my inner OCD hates it.
Using units on line-height is generally discouraged, but provides immediate
knowledge of what the computed value is. We’ll probably try to steer
away from this in the future.
In the future, we’ll likely use ems for type sizing, perhaps rems even, but not for anything else. This is also debatable on font sizes for inputs and the like. It’s just not how folks build pixel perfect sites.

That’s a bit all over and hopefully coherent enough. I’ll try to blog about these changes as they come up more, but I’m unsure how close 3.0 is and what that will all entail yet.

I would suggest anyone with strong feelings about this go and +1 this thread.

[Update] V3 roadmap oulined in V2.3 release blogpost makes no mention of adding support for ems.

[Update] Lots more information about Bootstrap V3 available in the pull request here including the following from Mark Otto:

We explored the use of rem units over pixels, but found little benefit
to offset the implications of their use. IE8 would still need a pixel
fallback, and that’s a lot of duplicate lines of code. Moreover, using
rems everywhere instead of pixels would exacerbate that problem.
Mixing rems and pixels doesn’t seem to make sense either right now.
However, we can and will continue to evaluate this in future releases.

Then more recently (in its comments):

I highly doubt we’ll be shipping with rems at this point. Changing
everything—beyond font-sizes—is a huge task and one that comes with
few benefits to offset that. Double the lines of code for font-sizes
aside, supporting rems in any other way seems tedious at best. That
said, we can always revisit in a future release. For now, we’re
sticking with pixels.

Having grown dissatisfied with a large number of Bootstrap’s features, not least of which is it’s lack of em-support, I strongly suggest looking at Susy if you just want grids, or Zurb Foundation 4 for the whole enchilada. Don’t let Bootstrap’s popularity cloud your judgement. Anyone can build something with Bootstrap, which is exactly its problem – it’s designed for people with minimal web-experience. Just because there are lots of McDonalds’ in the world doesn’t mean it’s a healthy place to eat.

[Edit] OK. This was a silly thing to say. Since I wrote this, I’ve used BS3 and it has upped its game considerably. I shouldn’t have made such a throwaway comment, but I still believe that it has made a bad decision in using pixels for font-sizing. As well as accessibility issues, ems are useful in other ways.

[Update] Looks like rems will be supported in V4 (Mdo quoted from here):

For those following along, we’ll be able to change from pixels to REMs
in v4 when we drop IE8 support. Can’t do much until then.

[Update Feb ’17] Bootstrap 4 is still in Alpha, but shows use of rems in its Typography docs, but doesn’t show use of rems in its layout docs.