CSS: Issue with rounding errors in Em units

Recently, I decided to update the CSS file for a website I'm working on by making most elements and fonts dynamic in size using the em unit instead of px. The new sizes seem to be working, but there's an issue that's been bothering me.

It seems that when I use the em unit for setting margins on a box (margin: 0.25em), I'm getting different results in Firefox compared to Internet Explorer. In Firefox, I get 4px on the top and left, and 3px on the right and bottom, based on a font size of 14px (which is actually 0.875em of the browser's default 16px font size).

As someone new to using em units, I might be missing something, but based on my calculations, 0.25em of 14px should be 3.5px. So, why the discrepancy between 3px and 4px? Shouldn't rounding be consistent? In Internet Explorer, the margins are actually reversed: 3px on the top and left, and 4px on the right and bottom. I even tried adjusting the value to 0.286em to avoid dealing with half pixels, but the results were still inconsistent between browsers.

Despite these pixel discrepancies, I have to admit that changing the browser's font size does a nice job of scaling everything smoothly.

Am I missing something important here or are both browsers experiencing some sort of bug? (I tested on IE8, FF3, and FF19.)

Answer №1

Consistent rounding for smooth scaling

When a browser scales a page, it may round values up or down inconsistently to achieve an overall balance in rounding operations.

For example, on mobile devices or desktop browsers with manual zooming, scaling a page changes integer values to floating-point values, affecting the positioning and size of elements in pixels. The browser must round these values to integers for accurate display, and inconsistent rounding is necessary for the sizes to add up correctly.

In a scenario with three divs each set to 100px, scaled to 2/3 the size, consistent rounding may result in the total width being off by a pixel. By inconsistently rounding some values up and others down, the sizes can add up accurately.

Not all browsers handle this rounding inconsistency well, with Firefox being noted for its effective handling of scaled pages. Other browsers may be improving in this aspect, but may not have been as proficient in the past.

Understanding this behavior is crucial for dealing with scaling issues in web design, particularly in cases involving absolute positioning or background images from sprites.

Answer №2

It's not so much that they are broken, but rather that various browsers handle pixel rounding differently.

If you want more consistent results, consider using rounded values that don't result in decimal numbers or use exact pixel measurements instead of ems when precision is crucial. Keep in mind that ems are meant for scalability and may not provide the level of accuracy needed in certain cases.

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