Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Make Reading on the Web Easier Using Safari 5 Reader
At some websites, the text is so tiny or has such crazy colors that even people with good eyesight complain. You could zoom in to enlarge the text, but how to change the color? A smart and convenient option will be to use the Safari Reader feature. However, it requires Mac OS X Leopard 10.5.8 and greater as well as the Safari 5 browser. There’s only one drawback – Reader doesn’t work at all webpages. If you see a Reader badge to the right of the Address Bar, it means that this page supports the Reader.
To use the Reader do one of the following actions:
• Click the Reader badge in the Address Bar.
• Click View, Enter Reader.
• Press Command+Shift+R.
As a result the web page text becomes readable, appearing in black on the white background, just like an ordinary MS Word document.
To change size of the Reader text either click the magnifying glass icon in the control bar at the bottom of the Reader or:
• Use the Command button and plus-minus signs
• Click View, Zoom In or Zoom Out.
• Use the Reader’s Control Bar.
The Reader has a Control Bar also enables you to email or print the text.