I frequently run into code posted to this site which I'd like to see in rendered form in order to quickly decide whether it fits my needs.
Opening a document just to do this often feels like a hassle. Are there any plans to add the ability to, for instance, just hover the cursor over a block of code or press some button below it and have a popup appear that shows what the output looks like?
This would be very similar to requesting MathJax support, which has been declined here as we're more interested in the code rather than the output. That doesn't mean the output is not important, of course.
Moreover, many of the code posted by users here contain errors which they don't know how to correct. Or, it contains customizations that are found outside of the default distribution(s).
Stack Overflow implemented something similar as a feature request, although limited in language scope. The extendability of (La)TeX as well as it's different compilers would be difficult to implement easily, if at all.
It's best if users can add an image of the output they see (or expect to see) as pictures are often worth a 1000 words.
z_1, m^2, p_0^2 in a TikZ diagram or something similar. In such cases, it doesn't really matter if the MathJax output doesn't exactly reproduce the LaTeX output, it would just circumvent cluttering up posts with underscores and circumflexes.
As @paul-gessler has commented, this has been asked several times before, and the community of TeX.SE is generally against such a feature, mostly because of problems with erroneous code, compatibility, or output that requires multiple files (like bibliographies).
However, I am strongly in favour of this feature, if it is implemented with a separate syntax. In this way a normal code block is be shown as code, which is indeed the most sensible behaviour for this site. However, if the user wants, the output can be shown as well. For example:
# \documentclass{article}
# \begin{document}
# Hello World
# \end{document}
Analogous to the syntax for block quotes.
This would save time for almost every time you post an answer where you want to include a screenshot of the output. Creating a new file, copying the MWE, compiling, making the screenshot, cropping, saving, uploading sometimes takes me more time than answering the question itself (for example with answers like "use \phantom") and I do this for practically every answer. Another use case is in-line code, like "the $\sum_{\alpha\in\{-5,-10\}\cup\mathbb{Z}^+}$ is not aligned with the $\sum_\beta$".
TeX.SE could support for example something analoguous to the texlive-full package in Ubuntu. This supports almost everything. If the result is different from what the OP or the answerer has on his own system, then he can choose to show a screenshot instead. If the question or answer depends on a specific engine, command line options, fonts installed, ..., then a screenshot can be used. But in the many, many cases where the setup doesn't matter, the users can benefit from a compilation feature.
.pngfiles are easy to add to a question or an answer), and i think that's the most reasonable thing to expect. but it is up to those who are answering questions to "do the right thing".\donotincludeinmaintoc, that will giveundefined control sequence, because the package defiing the command would not be loaded.