I have tried Firefox and Chrome and on Ubuntu 14.04 LTS, and there seems to be a sizeable chunk of Unicode characters that I cannot render.
What is the best course of action to diagnose the root cause and fix the missing characters?
Install package ttf-ancient-fonts.
sudo apt-get update sudo apt-get install ttf-ancient-fonts
In order to see the missing characters, you need to install a font which contains them.
I've found that Symbola contains the best range of characters.
.ttffonts - if you double click on then, you should see a font preview, click the "Install" button.
With the latest version (8.0.0) I'm able to see the dog symbol 🐕 and the slice of pizza symbol 🍕.
The most comprehensive font package I have found is
font-noto. Installing this has always fixed whatever font-display issues I was having:
sudo apt install fonts-noto
Then, restart your browser and everything should work as expected.
I tried to view the characters on my computer (See signs and symbols). Enter the Unicode-code in the search-window. It looks that the symbols mentioned are not rendered yet (These characters are in the Supplementary Multilingual Plane). See also http://www.ltg.ed.ac.uk/~richard/unicode-sample-3-2.html.