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HTMLayout %% length units

In short, %% units are "percentage from free space". They also known as flex units or relative length units.

The engine supports two forms of flex units: "spring coil" notation - 100%% and star notation - 1* and these forms are related as:

100%% = 1*

%% units are applicable to border, padding, margin, width and height CSS attributes only. HTMLayout will effectively compute them for all blocks in 'normal' flow. Thus width:100%% of floated block will not be interpreted at all (will be treated as undefined).

Formal definition of %% length units computation:

When allotting space among element attributes competing for space along axis, HTMLayout engine allot all values for attributes having other than %% lengths first, then divide up remaining available space among %% lengths. Each %% length receives a portion of the available free space that computed as a percentage from total sum of all %% units along axis. If total sum of all %% lengths along axis is less then 100 then 100%% value is used as a total sum of all %% units. Thus, if 100 pixels of space are available after the user agent allots pixel and percentage space, and the competing relative lengths are 10%%, 20%%, and 70%%, the 10%% will be alloted to 10 pixels, the 20%% will be alloted 20 pixels, and the 70%% will be alloted 70 pixels. If 100 pixels of space are available, and the competing relative lengths are 10%%, 20%%, and 30%%, the 10%% will be alloted to 10 pixels, the 20%% will be alloted 20 pixels, the 30%% will be alloted 30 pixels, and remaining 40 pixels will be left undistributed.

Example:

In the example below I am using HTML table to model layout which could be done by %% units without need to use tables. Some %% use cases cannot be reproduced even by HTML tables.

Imagine that four blocks below declared in CSS as:

width:100px
width:30%% width:70%% width:150px

Below is the same layout but with %% units use. Could be viewed properly only in HTMLayout engine so far:

width:100px
width:30%%
width:70%%
width:150px

Two "tables" above have same structure and dimensions of "cells" in HTMLayout engine.

And following example is just one DIV having left and right border set to 30%% and 70%% respectively.

border-left:solid tan 30%%;
border-right:solid tan 70%%;
border-top:solid tan 4px;
border-bottom:solid tan 4px;
width:50%;
background: gold

DIV above takes all available width of its container in HTMLayout engine.

To see examples of %% units alive download HTMLayoutDemo.zip - HTMLayout engine and its demo application with samples.

How to use it:

  1. Unzip the archive somewhere on disk drive of your Windows PC .
  2. Start /bin/browse.exe and open html files in samples folder of the distribution by using menu: File/Open of the application.

Samples explanation:

  • html_samples/percent_percent_units.htm - copy of this file.
  • html_samples/editor/index.htm - mockup of UI of sample application. %% units are used there to position elements horizontally and vertically.
  • html_samples/sidebars.htm - typical web page layout esciso.
  • html_samples/backgrounds.htm - sample of backgrounds supported by HTMLayout engine. Gradient backgrounds and extended tiling demos.

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