Π (Π) vs Π (Π)

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.

• Use the standard 7 bit ASCII. If its about math write out 'pi', 'Pi' or 'PI' (depending on context):
  area of circle = pi * r²
  Product = PIⁿ_(k-1)(x - x_k)

  This can also be 'extended' to include ASCII art drawings:

             n
  Product =  PI (x - xk)
            k-1
  
  or
             n
            ---
  Product = | | (x - x )
            | |       k
            k-1
  

  (Another example of this can be found at sigma notation) At some point in the future, MathML will make the above drawing much easier, however, today it is even less supported than the various character sets.

• Use an image. Many web sites dealing with math create .gifs for each equation to make certain that the image is displayed correctly. However, this has the problem that images don't display on text browsers. Furthermore, it often means downloading possibly hundreds of little images - one for each equation. If this option is used, take care to add the alt attribute to each image. (This option cannot be used on Everything - thus, I can offer no examples)

• Use the numeric form of &#(number);. For 'pi' this is Π. This has two disadvantages:
  1. It is hard to read when doing edits: Λωγωσ - oops, that should be omicron, not omega.
  2. On browsers that don't support this character set, this appears to be a '?'. Seeing the word '?????' does not convey any meaning. Looking at the source just shows a jumble of numbers.
  The only advantage is that on some browsers, this actually does work when the symbolic form does not. However, as a personal recommendation I strongly recommend against this because it fails to convey any information to many people.

• Use the symbolic form of &something;. For 'pi' this is &pi;. If you have to use extended characters, especially that of Greek, this style of characters is visible to everyone. Those that cannot read the literal π can still see the &pi; - which is still very readable. The only care that should be taken with this is in the context of <pre> tags where the width of the character on a browser that displays it properly is 1, while those that do not will have something between 4 (for &pi;) and 8 (for &omicron;) characters throwing off alignment.

(Note the title of this is: "&amp;Pi; (&Pi;) vs &amp;#928; (&#928;)")