Use em or rem font-size in Bootstrap 3

bootstrap-em.less

/**
 * Use em or rem font-size in Bootstrap 3
 */

@font-size-root: 14px;
@font-unit: 0rem; // Pick em or rem here


// Convert all variables to em

@font-size-base:  1 + @font-unit;
@font-size-large: 1.25 + @font-unit;
@font-size-small: 0.85 + @font-unit;

@font-size-h1:            2.6 + @font-unit;
@font-size-h2:            2.15 + @font-unit;
@font-size-h3:            1.7 + @font-unit;
@font-size-h4:            1.25 + @font-unit;
@font-size-h5:            @font-size-base;
@font-size-h6:            0.85 + @font-unit;

@line-height-computed:    (@font-size-base * @line-height-base);

@padding-base-vertical:     ((6px / @font-size-root) + @font-unit);
@padding-base-horizontal:   ((12px / @font-size-root) + @font-unit);

@padding-large-vertical:    ((10px / @font-size-root) + @font-unit);
@padding-large-horizontal:  ((16px / @font-size-root) + @font-unit);

@padding-small-vertical:    ((5px / @font-size-root) + @font-unit);
@padding-small-horizontal:  ((10px / @font-size-root) + @font-unit);

@padding-xs-vertical:       ((1px / @font-size-root) + @font-unit);
@padding-xs-horizontal:     ((5px / @font-size-root) + @font-unit);

@border-radius-base:        ((4px / @font-size-root) + @font-unit);
@border-radius-large:       ((6px / @font-size-root) + @font-unit);
@border-radius-small:       ((3px / @font-size-root) + @font-unit);

@caret-width-base:          ((4px / @font-size-root) + @font-unit);
@caret-width-large:         ((5px / @font-size-root) + @font-unit);

//** Default `.form-control` height
@input-height-base:         (@line-height-computed + (@padding-base-vertical * 2));
//** Large `.form-control` height
@input-height-large:        ((@font-size-large * @line-height-large) + (@padding-large-vertical * 2));
//** Small `.form-control` height
@input-height-small:        ((@font-size-small * @line-height-small) + (@padding-small-vertical * 2));


// Set root font size

html {
    font-size: @font-size-root;
}


// Change styling for elements that mix pixels and ems

pre {
  font-size: ( @font-size-base - (1 / @font-size-root) );
}

.form-control-feedback {
  width: calc(@input-height-base ~" + 2px");
  height: calc(@input-height-base ~" + 2px");
}
.input-lg + .form-control-feedback {
  width: calc(@input-height-large ~" + 2px");
  height: calc(@input-height-large ~" + 2px");
}
.input-sm + .form-control-feedback {
  width: calc(@input-height-small ~" + 2px");
  height: calc(@input-height-small ~" + 2px");
}

.form-control {
    height: calc(@input-height-base ~" + 2px");
}

.form-horizontal {
    .radio, .checkbox, .radio-inline, .checkbox-inline {
        padding: @padding-base-vertical;
        border: 1px solid transparent;
    }
    .radio, .checkbox {
        min-height: calc((@line-height-computed + @padding-base-vertical) ~"+ 1px");
    }
    .form-control-static {
        padding-top: @padding-base-vertical;
        border-top: 1px solid transparent;
        padding-bottom: @padding-base-vertical;
        border-bottom: 1px solid transparent;
    }

    @media (min-width: @screen-sm-min) {
        .control-label {
            padding-top: @padding-base-vertical;
            border-top: 1px solid transparent;
        }
    }
}

Unfortunately that gist breaks all rules where @line-height-computed is used as a value for margins and paddings as those properties take the element's own font-size as the base size.

Such rules occur 31 times in Bootstrap 3.3.2 and that makes this gist useless unless all those places are patched.

And even after overriding those rules such patching need to be maintained for every new release of Bootstrap.

Hopefully BS4 will have ems/rems as base units

Are there any plans to use rems in BS4 ? We have a working rems version that we use in our company - we could contribute it to BS4. Do you know who maintains the backlog or planning of BS4 ?

More info available in a stackoverflow topic and a Github Issue

For those following along, we'll be able to change from pixels to REMs in v4 when we drop IE8 support

@pribilinskiy, could you please elaborate on why @line-height-computed here breaks those margin and padding rules? I'm not against patching my copy of bootstrap. How to make it work with this gist?

@koddo, because old @line-height-computed was already applied to the rules that use it, you need to extract all of them and add it to the gist.

Where are variables @line-height-large and @line-height-small ?

does this convert all px in bootstrap into ems or just some controls?