Colspan>1 width redistribution
TD with colspan>1 redistributes its min/max/percent widths to the spanned cells.
Creating explainable tests with percentage cells is complicated. Cell's percentage cannot be observed directly, but must be inferred from td+table widths. Rules that govern relationship between table grid width and percentage cell width are:
 Rule#1, Each percentage column sets the floor for maximum table width. That table width is column.min_width / column.percent * 100.
Ex: 20px wide column, with 20% width, implies that the entire table must be at least 100px wide.  Rule#2, Sum of all percentages, combined with sum of widths of all nonpercentages sets the floor maximum table widths.
Let P% be sum of all percentages, and Fpx sum of widths of all nonpercentage columns. Maximum table width = Fpx * (1 + (100P%)/P%).
Ex: if percentage columns are 20%, and nonpercentage columns are 100px, implies that that 80% is at least 100px, and that the entire table is at least 125px.
Test design
All examples have borderspacing:8, td.padding:0.
Tests are mostly tables with 2 rows. 1st row are the columns, 2nd row is the colspan>1 column being distributed. Most test's colspan>1 cell colspan encloses all cells except the last one. Each test is accompanied by test description. Test description contains:
 First line describes the test geometry: cell:css/min/max. C1:20%/20px means width:20%, intrinsic min width 20px. If max is omitted, min == max.
 Second line describes algorithm being tested in detail
 Third line describes computation that generates the result.
 Red paragraphs are major browser disagreements
Colspan>1 cell's percentage distribution auto tables
Rules
 Percentages can only be redistributed to nonpercentage cells.
 If percentage does not get redistributed, treat colspan>1 cell width as Auto
 If all columns are empty (no max width), redistribute percentage evenly.
C0:10%/20px C1:10%/20px C2:auto No colspan>1 cells, shows what table looks like without colspan>1 cell distribution. Table width by rule#1, 20px/0.1(10%) + 4*8 => 232px. Excess table width is distributed to auto cell.
x 
x 
x 
Colspan>1:40%/Auto C0:10%/20px C1:10%/20px C2:auto Percentage does not get redistributed because all columns are percentages. Table width by rule#1 same as previous example.
x 
x 
x 
40% 
Colspan>1:20%/100px C0:Auto/0 C1:Auto/0 C2:Auto/Auto Percentage gets redistributed evenly to empty cells. Each cell gets 10%, then (1008)/2=>46px min width. Table min width is 46/0.1(10%) + 4*8 => 460 + 32 => 492
Chrome Legacy is wrong, 1st span cell gets all the width. FF/Edge agree.
x  
100px 
Colspan>1: 19%/200px colspan>1 cell, C0: 20%.20px, C1: 80px/80px Percentage is not getting redistributed, because column% > colspan>1%
20 
80 
208 
Colspan>1 cell's percentage distribution fixed tables
Colspan cells distribute width over col widths.
Rules
 Percentages are not distributed to fixed columns.
 Percentages are not distributed to percentage columns.
 Percentages are distributed to auto columns, each column gets distributed%/colspan percent.
Auto column distribution auto colums get percentage widths distributed evenly. C0/C1 become 25% columns. Assignable table size is 4004*8=368. Column size is 25% of 368 = 92px
FF disagrees in how borderspacing is handled and ends up with slightly different cell widths.
50%  a  
a  aa  a 
Fixed column distribution fixed columns do not get percentage distribution.
50%  a  
a  a  a 
Percentage column distribution percentage columns do not get percentage distribution.
50%  a  
a  a  a 
Distribute over percentage/auto/fixed mix cells Collspanned TD distributes 50%. Auto TD gets 50%/(colspanned span = 4): 12.5% 12.5%* 400px is 50.
50%  a  
a  a  a  a 
Colspan>1 cell's minimum width distribution
All unconstrained columns
Colspan>1: Auto/300px C0:Auto/75pxpx C1:Auto:25px; C2:20px/Auto Basic distribution over auto columns, cells grow evenly. Distribute 3008=292 proportional to maxwidth. C0 gets 75/100*292=219.
75px 
25px 
x 
300px min 
Colspan>1: 260px/300px C0:Auto/100px C1:Auto/100px C2:Auto/Auto Colspan>1 cell min width wins over css width. Table width same as previous example.
100px 
100px 
x 
300px min 
All fixed columns
Colspan>1: 260/300 C0:100/50/100 C1:100/100 C2:20/Auto Distribute surplus proportional to max width. Colspan>1 is distributing 292, surplus 92, each column gets 46.
50 50 
100px 
x 
300px min 
Colspan>1: 300/300 C0:100/25 C1:100/75 C2:20/Auto Column minimum width is less than its CSS width. Just like last example, both columns get 46.
25px 
75px 
x 
300px min 
Colspan>1: 300/300 C0:20/100 C1:100/75 C2:20/Auto First column's min width > css width. Like last example, each column minimum width increases in proportion to the max.
Chrome differs from FF/Edge. It distributes minwidth in proportion to css width, not max width
20/100px 
40/100px 
x 
300px min 
All percent columns
 min width never gets smaller than it started.
 min width becomes cell.percent/cells.percent * colspan>1_cell.min_width
 max width becomes cell.percent/cells.percent * colspan>1_cell.max_width
Colspan>1:Auto/300px C0:25%/50 C1:25%/30 C2:20/Auto Colspan>1 min width gets distributed proportional to percentage (not min/max width). Columns get min_width = 146, causes table width of 146/0.25 + 4*8 = 616.
25%/50 
25%/30 
x 
300px min 
Colspan>1:Auto/400px C0:20%/50 C1:60%/50 C2:20/Auto. Colspan>1 min width gets distributed proportional to percentage (not min/max width). Cells get 98/294, table by rule#1 98px/0.2 + 32 = 522
Edge disagrees, table is 870
20%/50px 
60%/50px 
x 
400px min 
Colspan>1:Auto/468 C0:50%/150 C1:30%/150 C2:20/Auto This tests conflict resolution where minwidth > redistributed min width. Colspanned redistribution: distributed 4688 = 460 over C0/C1 Colspanned 468px needs to distribute 460px over C0/C1. C0 percent size is 50% * 460 = 230 C1 percent size is 30% * 460 = 138, defaults to min size of 150 Column 1 size is 230 + 50%/80% * 80 = 280px Column 2 size is 150 + 30%/80% * 80 = 180px Column 3 remains 20px Assignable table inline size C1 dominates estimate: 180/30% + 4*8 = 632 Compute final column widths from assignable table size: C0 = 50% of 600 = 300 C1 = 30% of 600 = 180 , C2 gets the remaining 120
50%/150px 
30%/150px 
x 
408px min 
Colspan>1:Auto/400px C0:50%/75px/125px, C1:30%/75px/125px C2:20px/Auto Colspan>1 cell distribution over different percentages. 4008px min width gets redistributed as 245/147 (no min width limits)
Edge is different, table is 685 instead of 522.
50%/75 /125 
30%/75 /125 
x 
400px min 
Auto/fixed mix columns
Colspan>1: Auto/500; C0: Auto/40 C1:150/100 Colspan>1 excess min: 5001408=352; excess max: 5001908=302 Initial min/max C0:40/40, C1:100/150 Excess is distributed to Auto C0:392/342, C1:100/150 Assignable width is 492. C0 gets 342, C1 get max:150 Chrome Legacy does something very different to arrive at the same answer. Initial min/max is the same. Min gets distributed to C0 and C1 in proportion to max_width: C0:342/40 C1:150/150, Min also gets clamped to css width. This is the key difference betweeen NG and Legacy code. NG cannot clamp to css width, because it is not available. Max gets distributed to both in proportion to max_width: C0:342/103 C1:150/389 Assignable table width is: 492 (sum of min), min gets used as final width. Analysis: colspan>1 cell distribution over fixed/auto. Legacy incorrectly distributes minmax over both fixed and auto cells in proportion to max width,and clamps min to css width. This width becomes final width for auto tables without any further distribution. FF distributes min only over auto, and max over auto. Final distribution uses Guess 2 rules: start off with Auto(min), Fixed(max), distribute excess to Auto. Ends up with the same result as Legacy.
Auto 
150 
500 
C0:70/Auto C1:70/Auto C2:Auto/50 This testcase demonstrates how Legacy Chrome handling of min widths is broken. There are 2 fixed columns, and 1 auto. The width of fixed columns should be the same, but is not. The problem happens when minwidth of fixed columns is less than fixed width, and there are auto columns.
Chrome Legacy incorrectly assigns minimum width to C0/C1 to css width of 70. It does compute assignable table width correctly as 166 px. This violates the invariant: assignable table width >= sum of min widths. Chrome resolves invariant violation by truncating one fixed width column below its minimum width.
0  0  50 
Percentage/fixed mix columns
The Legacy code that deals with this is confusing, it tries to redistribute minimum width in proportion to max width, unless there are auto cells. All the browsers disagree on final widths. Some things everyone agrees on:
Colspan>1:N/A C0:60%/100 C2:100/100 An example what table looks like without colspan>1 distribution. Table width by rule#2: (60/(10060) +1) * 100 = 250 + 24 = 274
60% 
100px 
Colspan>1:N/A C0:50%/100 C1:50%/100 C2:100/100 Total column percent is 100%, forcing maximum table grid width by rule #2 to infinity. Table css width is 1px, overriding grid max width. Final column width is column's minimum width.
50% 
50% 
100px 
Colspan>1:N/A C0:50%/100 C1:50%/100 C2:100/100 Same example as above, but table width is auto. Table grows to size of containing block.
50% 
50% 
100px 
Colspan>1: Auto/200px C0:40%/20 C1:50/50 C2:100/20 Table css width is 1px. This forces grid minimum. FF: %ge cell gets its percentage width resolved wrt distributing cell. fixed cells get remaining excess width redistributed in proportion to max size. Chrome algorithm: distributes proportionally to min_width. The problem with this is that 40% cell will end up with 0 if no min width.
Edge distributes all the width to the %ge cell, Chrome distributes proportionally to min width.
40% 
50 
20 
208px min 
Colspan>1 cell's maximum width distribution
Colspan: constrained. Col: constrained Constrained colspan redistributes widths over constrained cols.
50  50 
200 
Colspan: unconstrained. Col: constrained Colspan maxwidth does not get redistributed.
50  50 
50 50 50 50 
Colspan: unconstrained, Col: constrained/% Colspanned col distributes width over %ge column. maxguess is 50px + 200px*50% = 150px distributable_size is 50px %ge column gets percentage_size + distributable_size = 150px Later, this makes table 300px wide.
FF, Legacy, and TablesNG all disagree about the correct widths. FF feels more like the right answer, but I can't figure out the math behind it.
50  50 
50 50 50 50 
Merging cell widths into column widths
What happens when different types of cells get merged into a single column?
C0:0 25%/50px C0:1 50%/40px C0:0 alone would imply table width of 50/.25 = 200px. C0:1 alone would imply table width of 40/.5 = 80px. But, largest percentage and largest min width win, so the table width is 50/.5 = 100px.
25%/50px 
Auto 
50%/40 
Auto 
Merging fixed and percentage Column that contains both percent and fixed cells is treated as percent.
25%/100 
50px fixed  auto 
150  Auto  Auto 
Order of colspan>1 cell redistribution Initial minmax of all cells is 50px. C0:1 redistributes 200 C1:2 redistributes 400 Chrome distributes righttoleft: C1:2 distributes 400, and C1 and C2 both get 200 When C0:1 turns come, C0+C1 are already long enough, nothing to distirbute
In testcase /tables/mozilla_expected_failures/core/col_span2.html distribution righttoleft is described as a bug. Firefox and Edge both distribute left to right. Chrome is only one that distributes right to left. Firefox distribution is not cumulative, it uses original minmax to compute final widths. Edge agrees with TablesNG. But, mozilla is stable for row reordering.
200 
50 

50 
400 

Auto  Auto 
Auto 
400 
50 

50 
200 

Auto  Auto 
Auto 
Order of colspan>1 cell redistribution Similar to previous testcase, but colspan is 3, not 2.
In testcase /tables/mozilla_expected_failures/core/col_span2.html distribution righttoleft is described as a bug. Firefox and Edge both distribute left to right. TablesNG matches Edge/FF.
300 
50 

50 
600 

Auto  Auto 
Auto 
Auto 