Using fieldsets for decoration purpose is a bad practice as it leads to invalid HTML and potential accessibility issues. In the pattern library case, it's also annoying because the fieldset's default styles prevent an appropriate preview for most high level patterns.
Solution
Do not use fieldsets to wrap patterns previews. Use a simple div and a header instead.
Problem
Using fieldsets for decoration purpose is a bad practice as it leads to invalid HTML and potential accessibility issues. In the pattern library case, it's also annoying because the fieldset's default styles prevent an appropriate preview for most high level patterns.
Solution
Do not use fieldsets to wrap patterns previews. Use a simple div and a header instead.