A few people are noticing after installing Lion that Safari may render fonts in Web sites incorrectly, and show some or all of the text as a series of square blocks with the capital letter A in them. When testing the problem in other browsers, such as Firefox, the sites appear fine. The problem does not happen to all Web sites, but instead seems to crop up only for specific ones, indicating an issue with how the rendering of specific HTML or styling code is being interpreted for those sites.
Users who have had this problem have tried resetting Safari cookies and browser caches, but have not had any any luck with these measures, indicating the problem is neither with site settings nor any conflicts with the browser's cache.
One cause for this problem is that there may be a conflict between various fonts you have on the system, or there may be an issue with the font cache or other system caches. Therefore, to manage these problems you can either validate fonts using Font Book to see if there are any font conflicts, or run various cleaning routines to clear system caches.
Apple's Font Book application can be used to manage system fonts, including turning them on and off, as well as validating them to see if there are any conflicts. To run a Font validation, open Font Book and select your fonts (press Command-A to select them all), followed by choosing Validate Fonts from the File menu. The system will check each font and show you whether they have passed, or whether minor or major problems were found. After the routine has run, you can manage the problematic fonts by either removing them or disabling them, or otherwise fixing the specific issue at hand.
Font caches can be cleared by using a utility like OnyX. While you can run a full to reset a number of system fonts and temporary files, to specifically target font caches just install OnyX or a similar tool (be sure it supports OS X Lion), then boot to Safe Mode by restarting with the Shift key held down, and run the tool to specifically delete the font caches (the location of this option will depend on the tool). Then reboot the system normally and try viewing the problematic Web pages in your browser again.
Clearing caches and booting to Safe Mode has helped the majority of people experiencing this problem; however, for a few people the problem returns after a while, indicating that either Safari itself or the system's font configuration may be actively corrupting the font cache.
MacFixIt reader B. Jefferson Le Blanc wrote in describing such an issue:
I have a test partition with Lion installed. One of the things that has discouraged me from moving to it full time is the return of a display font problem in Safari 5.1. If I clean the font caches the problem is resolved until the following restart of Lion and Safari. It seems clear that Safari 5.1 is messing up the font cache in some way.
If this happens, then unfortunately the approach would be to systematically disable system fonts using Font Book until the problem goes away, and then remove the specific fonts that are contributing to the problem. Since some systems can have hundreds of fonts installed, it may be best to lump them into a number of font collections and then disable the individual collections to better isolate the problematic fonts. To create a collection, just select a group of fonts, drag them to the "Collections" column in Font Book, and then optionally name that collection.