The articles are very interesting and can be found here:
The Magic Style Guide Part 1: http://www.wizards.com/Magic/Magazine/Article.aspx?x=mtgcom/daily/mc3
The Magic Style Guide Part 1.5: http://www.wizards.com/magic/magazine/Article.aspx?x=mtgcom/daily/mc4
The Magic Style Guide Part 2: https://www.wizards.com/magic/magazine/Article.aspx?x=mtgcom/daily/mc5
Magic: The Gathering started using a Style Guide in it's "Tempest" set, the first set of cards in the "Tempest" block. Tempest is the 12th card set of Magic: The Gathering, released in October 1997. This means that Wizards went 12 sets and 4 years without using a Style Guide - which produced some issues for it's artists and players.
The following art is from the "Ice Age" block, the 11th set released in 1995. Upon hearing the name "Ice Age" appropriate imagery comes to mind, which had worked for previous card sets. However, Ice Age had a race of people known as the "Balduvians" who resided in the universe and, without a style guide, artistic license was taken with their appearance. The following is artwork that appeared on cards from different artists depicting the Balduvians.
Although there are some similarities, the variance is still wild in some artwork. This is when a Style Guide would have created a much more unified look.
Mirroden Notes
The Following is a set of written guides for the art style of the "Mirroden" Block.From looking at the rules, suddenly what is obvious is thrown out the window and new ideas need to be created to compliment the world created. I will challenge myself to create artwork adhering to these rules.