Techniques for WCAG 2.0

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SCR34: Calculating size and position in a way that scales with text size


Client-side scripting.

This technique relates to:

User Agent and Assistive Technology Support Notes

Calculating size and position can be complex and different browsers can produce different results. This can occur when the CSS styling mixes padding, margins and widths for an object, or when it mixes an offset and plain value, e.g., offsetWidth and width. Some of these behave differently in reaction to zooming. See MSDN: Fix the Box Instead of Thinking Outside It for an explanation of the way that Internet Explorer 6 and later differ from earlier versions of Internet Explorer.


The objective of this technique is to calculate the size and position of elements in a way that will scale appropriately as the text size is scaled.

There are four properties in JavaScript that help determine the size and position of elements:

Calculating the height and width using offsetHeight and offsetWidth is straightforward, but when calculating an object's left and top position as absolute values, we need to consider the parent element. The calculatePosition function below iterates through all of an element's parent nodes to give a final value. The function takes two parameters; objElement (the name of the element in question), and the offset property (offsetLeft or offsetTop):


Example 1

The Javascript function:

Example Code:

function calculatePosition(objElement, strOffset)
    var iOffset = 0;

    if (objElement.offsetParent)
            iOffset += objElement[strOffset];
            objElement = objElement.offsetParent;
        } while (objElement);

    return iOffset;

The following example illustrates using the function above by aligning an object beneath a reference object, the same distance from the left:

Example Code:

// Get a reference object
var objReference = document.getElementById('refobject');
// Get the object to be aligned
var objAlign = document.getElementById('lineup'); = 'absolute'; = calculatePosition(objReference, 'offsetLeft') + 'px'; = calculatePosition(objReference, 'offsetTop') + objReference.offsetHeight + 'px'; 


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  1. Open a page that is designed to adjust container sizes as text size changes.

  2. Increase the text size up to 200% using the browser's text size adjustment (not the zoom feature).

  3. Examine the text to ensure the text container size is adjusted to accommodate the size of the text.

  4. Ensure that no text is "clipped" or has disappeared as a result of the increase in text size.

Expected Results