It is the unfortunate case that there are some
browsers that either don't have the
font to display the characters, or don't even support the characters. There is little that can be done to rectify it, even if every
browser that came out from this point on supported it every extended html character - it is still up to the owners of the machines to
upgrade the
browser version and install the fonts or
switch browsers. This simply is not going to happen and it can't be
mandated.
At the core of writing is the goal of communicating ideas. These ideas are attached to symbols that are recognized by the reader. If for some reason an author is writing about circles and wants to include the character 'Pi' there are four choices that the author can use.
(Note the title of this is: "Π (Π) vs Π (Π)")