The other answers are out-of-date. As of 2012, beautiful math is easy to write and render. The technology is called MathJax. You can see it in quiet action on MathOverflow and hundreds of math blogs.
MathJax is an open source JavaScript display engine for mathematics that works in all modern browsers. No more setup for readers. No more browser plugins. No more font installations… It just works.
Mathjax is reliable and unobtrusive, so you just need to write the math. You do so in Tex (Latex), a concise syntax with which most scientists and mathematicians are familiar (and have shared decades of good tutorials). For Mathjax, you simply write Tex code in-line in your HTML between double dollar signs, eg.
When $$a
e 0$$, there are two solutions to $$ax^2 + bx + c = 0$$ and they are $$x = {-b pm sqrt{b^2-4ac} over 2a}.$$
To use Mathjax to render your math, put a Javascript line in your HTML header:
<script type="text/javascript" src="http://cdn.mathjax.org/mathjax/latest/MathJax.js"></script>
If you publish on a platform such as Wordpress, Tumblr or Blogger there are plug-ins in their galleries to do this (Wordpress).
How does Mathjax render math? With Javascript it renders your math to beautiful HTML and CSS (remarkably resembling Latex) in a fraction of a second. If a browser supports MathML, it can render math through that too, but that's not important. It's a popular success because the end-user workflow is easy, not because of the technology behind it.
You can choose to use Mathjax (over png images) on Wikipedia if you have an account. Look for Special:Preferences / Appearance.
MathML is ridiculous. It's neither human-readable nor human-writable (the quadratic equation takes 800 characters - it's 50 in Tex). It's just another pointless XML language . Thankfully, it's obsolete before most browsers support it. It doesn't even look as good as Tex or Mathjax's HTML-CSS!
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