When creating a web page, not all unicode characters are always available across default installation of various operating systems. Mostly because lack of fonts, or problematic fallback font system.
See for example, this presentation has many missing character in chrome on default Windows 7 installation.
Is there a caniuse.com-like website for unicode fonts availability?
(Yes, it’s always possible to embed free font in your document a la
<!DOCTYPE html>
<html>
<header>
<style>
@font-face {
font-family: 'Symbola';
src: url(
'https://github.com/laughinghan/mathquill/blob/master/font/Symbola.ttf?raw=true'
);
}
body {font-family: 'Symbola';}
</style>
</header>
<body>
<h1>☻ ⚭ ⚮ ⚯</h1>
<p>⚀ ⚁ ⚂ ⚃
⚄ ⚅ ⚆ ⎘</p>
</body>
</html>
but you might want your website to work without prior 870k font fetched, or you want it to work with email, or older browsers, etc)
I don’t thin there is any site of the caniuse.com type for characters; it would be a rather large site, and maintaining it would be a nightmare. I quickly souped up a short section “Which characters can I use” that I added to my Guide to using special characters in HTML, but it’s really just a few hints.
The good news is that nowadays the issue is mostly font support to characters, and this can often be handled by setting up a suitable
font-familylist in CSS. Browsers cannot always pick up a character from a font unless the web page author gives a helping hand that way.