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The default (original) box model used by browsers leaves a lot to be desired. When you specify something like:
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| language | css |
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| invalidmacro |
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the actual width of the element is padding + border + width. The same applies to height: padding + border + height. This makes defining the actual size of element very hard because adding a border means decreasing the size if the element's size needs to be constant. It also causes huge problems with things like "width: 100%" because those cannot have border nor padding: it would make the element exceed the 100% width causing those annoying scroll bars.
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The script used for this is in _core.scss, and applies the following:
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| language | css |
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| invalidmacro |
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This sets all tags to use border-box sizing by default, and makes it inherit so that changes made locally (should) percolate through.
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