125px is the default width of ordinary asp:TextBoxes in Internet Explorer, that's why we have used this number. Setting the width style explicitly is required by the more complex HTML code, which our textboxes have (which in turn is implied by their features and behavior).
Recently we optimized the rendering of our textboxes, so in some cases it became a lot simpler and the inline width style is not required, however, we have left it for backwards compatibility and to avoid scenarios in which one has two Telerik textboxes one by the other and suddenly they appear with different dimensions due to inherited styles.
At any rate, we may reconsider changing this in the future.
the Telerik team
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