What could cause the same browser on different PCs to render the same HTML differently?

Question

I'm stumped. A client and I are both running Firefox 3.0.12 on XP machines. We view the same page in the same browser on different machines and have different results. Note: All IE browsers are consistent on both of machines, and my FF 3.5.1 is consistent with my FF 3.0.12. (He does not have FF 3.5.x installed to compare with).

There is a 1 pixel difference that causes his header div (all images, no text) to wrap. Mine is fine. If I expand his header by 1 px (via Firebug css editing), it works.

So my question is what other variables are there that affect rendering??

I'm trying to think "outside the box" because it seems that it should work exactly the same.

We cleared the browser cache. I did a diff on the HTML source, and other than a javascript datetime stamp and a javascript tracking code var, the sources are identical.

I know this question would be helpful with source, but my client specifically does not want this page live on the internet yet. Also I verified the issue via a webex session. At first, I couldn't believe that we would have different results.

Anyone else come across something like this before? What should I check out / debug next?

Edit: There are a lot of suggestions pointing to fonts, but the header uses only images. Well, that's not completely true, there are divs that are part of a collapsible menu, but all of that is set to display:none for initially rendering, so I'm thinking that it's something else.

Edit 2: I've changed all sorts of fonts and sizes (both the system default font settings and within Firefox) on my computer to try to replicate the problem. I changed the rendering style (standard vs. clear type). Nada. The problem is almost definitely not font related. I'll have to see if I can get some more access to the client's computer to replicate the problem.

Answer 1

DIfferent operating systems have different font rendering engines. Vastly different. More than enough to cause at least a 1 pixel difference, depending on how you set up your CSS.

Answer 2

I know my FF3.0 makes everything look different than other people's because I have it set in Preferences to use a minimum font size of 16. I dont get fine print, I get a pile of legible writing.

Also, being on a Mac, the default fonts are sans serif, whereas on windows everything is serif, which also can change the font widths and heights even further.

If anything on your page is specified with "em" or "ex" units, they are dependent on font size.

Answer 3

One thing that comes to mind is installed fonts. If you have a font that the client doesn't there could be a 1 pixel difference in a computed size. FF and IE could be rounding differently, which would explain why they behave differently.

Answer 4

I'd compare plugins/addons.

Answer 5

Since we can't take a look at the site in our own browsers, here is the process that I would use to try to nail down the issue:

• Start removing code blocks piece by piece until the two browsers look the same.

• Whichever block of code you last removed that made everything line up perfectly (even if you are missing a chunk from the body), that's near where the culprit is.

• Since you've found the culprit, place all of the code back. Start messing around with the fonts first off. Change fonts, sizes, remove text, etc. until you can find a condition under which the browsers are the same. If it's not a font issue, start messing around with other parts of it until you've found the condition that matches it up.

• Once you've done that, you will know your issue and you can work around it.

Answer 6

How about your font smoothing settings - are they the same (none/antialias/ClearType)? That can change the width of a given text, and might just be what you're looking for.

Answer 7

Are you both using the exact same screen size? Does firefox try to scale the fonts to the apparent size of the person viewing the page, or to a specific number of pixels?

Answer 8

IF you're using Firefox make sure you hit Ctrl-0 to default the zoom level.

Answer 9

I mostly agree with all the "font guys" further up, but if that doesn't help try checking the HTTP headers since some headers might be added by proxies and such between the server and your client. But try the font suggestions first...

Good luck ;)

Answer 10

My first guess is something to do with the window, I don't recall this ever happening in firefox but try resizing the window anyway. It is also possible that there actually is something 1px wide taking up that space, such as a hidden frame or whatnot. These might be completely off the mark since I don't know how you actually made the thing.

A possible workaround is displaying one of the images as a background to a div instead of using an img tag, this'll cause it to simply be truncated instead of wrapping if it's 1px off.