At great personal irritation, I've learned some things about printer margins. I wasn't able to Google up this trivia or easily discern it from any documentation, so I will share it with you now.
When you create a new Letter-sized document in Illustrator CS3 and do Show Page Tiling, you'll see a dotted line showing margins ¼" from the left, top and right, and ½" from the bottom. These margins are advisory only: they are intended to show the part of the page that your printer can't actually print on. But your printer probably doesn't have such huge margins. In this modern world, 0.17" or so is more likely. So how do you change that?
First, you have to configure a printer! Even if you don't have a printer attached to the machine you're running Illustrator on, you have to pretend that you do. To fake it out, go to System Preferences / Print & Fax, click on the plus sign, select Line Printer Daemon; Address: localhost; Print Using: Select Printer Software... then pick a printer.
Now go into Illustrator's File / Print dialog and select that printer. In the page preview on the lower left, you'll now see margin outlines. Click Done.
These margin settings will be saved with your .ai file, and will show up even if you don't have a printer configured, but you can't change them without having a printer configured! So that's awesome.
Now, at this point, chances are you'll see that the margin outlines are in a nonsensical place like the left image below, with the margins lying off the left and top of the page. To reposition them to a sane orientation (the right image) you need to use the Page Tool, which is an option-click mode of the Hand Tool. It is well hidden and hard to use!
Having adjusted with the Hand Tool, you can go back to the Print dialog and save this as a preset, so that you don't have to do that every time you create a new document.
Again, these margins are only advisory, and don't affect the page origin. If you have ink 2" from the left edge of the document, it will show up 2" from the edge of the paper, regardless of the margins that are set in Illustrator. They're just like guides, I guess.
On Safari's Print dialog, the Paper Size menu lets you create a custom paper size with custom margins, but don't bother, because it doesn't work. Safari completely ignores those margins at all times. Safari always prints with ¼" margins on left, top and right, and a ¾" margin on the bottom, no matter what printer you have selected, or what custom page size you have selected.
Unlike in Illustrator, those margins are not advisory. The top-leftmost ink on a page in Safari is printed inside those margins, at ¼" from the top and left edges of the page. Even if your printer can drop ink at 0.1" from the left edge, or at ½" from the bottom edge, there is no way to get Safari to render into that area, even if your CSS set the margin, padding and border on the "html" and "body" elements to 0. Safari treats those CSS measurements as starting inside the hardcoded printer margins.
Also: if your document is wider than 8" (on an 8½" page), Safari will proportionally scale your document to fit when printing. So if you've carefully specified in CSS that some page element should be exactly "1in" square, but your page happened to exceed the hardcoded 8", then your square will come out some random amount smaller.
For added inconsistency: if your document is taller than 10" (on an 11" page), it will just be truncated at exactly 10", not scaled.