bootstrap 3 input-group 100% width

Question

In the bootstrap documentation, they have input groups that span 100% width with no additional markup: http://getbootstrap.com/components/#input-groups

Answer 1

input-group has display: table css property, while the has display: table-cell css property.

 
   
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According to HERE, under "Important Style Rules for Tables",

Width works on table cells just about how you would think it does, except when there is 
some kind of conflict. For instance if you tell the table itself to be 400px wide then the 
first cell of a three-cell row to be 100px wide and leave the others alone, that first cell
will be 100px wide and the other two will split up the remaining space. 

it explains that if the width of the child element that has table-cell is defined (let's say width: 100px), then the next child element with table-cell property, although not defined, will fill in the rest of the space.