diff --git a/components/layout_2020/flow/mod.rs b/components/layout_2020/flow/mod.rs
index e5695655296..e04f24e9c85 100644
--- a/components/layout_2020/flow/mod.rs
+++ b/components/layout_2020/flow/mod.rs
@@ -1142,12 +1142,15 @@ fn solve_inline_margins_for_in_flow_block_level(
pbm: &PaddingBorderMargin,
inline_size: Length,
) -> (Length, Length) {
- let available = containing_block.inline_size - pbm.padding_border_sums.inline - inline_size;
- match (pbm.margin.inline_start, pbm.margin.inline_end) {
- (LengthOrAuto::Auto, LengthOrAuto::Auto) => (available / 2., available / 2.),
- (LengthOrAuto::Auto, LengthOrAuto::LengthPercentage(end)) => (available - end, end),
- (LengthOrAuto::LengthPercentage(start), _) => (start, available - start),
- }
+ let free_space = containing_block.inline_size - pbm.padding_border_sums.inline - inline_size;
+ let margin_inline_start = match (pbm.margin.inline_start, pbm.margin.inline_end) {
+ (LengthOrAuto::Auto, LengthOrAuto::Auto) => Length::zero().max(free_space / 2.),
+ (LengthOrAuto::Auto, LengthOrAuto::LengthPercentage(end)) => {
+ Length::zero().max(free_space - end)
+ },
+ (LengthOrAuto::LengthPercentage(start), _) => start,
+ };
+ (margin_inline_start, free_space - margin_inline_start)
}
/// State that we maintain when placing blocks.
diff --git a/tests/wpt/meta/MANIFEST.json b/tests/wpt/meta/MANIFEST.json
index 206c345fd72..f70d2aa346b 100644
--- a/tests/wpt/meta/MANIFEST.json
+++ b/tests/wpt/meta/MANIFEST.json
@@ -73328,6 +73328,19 @@
{}
]
],
+ "margin-auto-on-block-box.html": [
+ "1afc98a91322feab5fe249b34c26274468fb0b22",
+ [
+ null,
+ [
+ [
+ "/css/CSS2/margin-padding-clear/margin-auto-on-block-box-ref.html",
+ "=="
+ ]
+ ],
+ {}
+ ]
+ ],
"margin-backgrounds-002.xht": [
"75c0211ca4b9c541eddb4b2698efd185574f0df4",
[
@@ -365406,6 +365419,10 @@
"287a58e369391edd2c3c3c6bcb6c5da4292ecb80",
[]
],
+ "margin-auto-on-block-box-ref.html": [
+ "efc41cff10c179da1a3cfe0da1cfea72530fa7d9",
+ []
+ ],
"margin-border-padding-001-ref.xht": [
"0578570aaef4f6b46557feb12b7368e0008dbb5b",
[]
diff --git a/tests/wpt/tests/css/CSS2/margin-padding-clear/margin-auto-on-block-box-ref.html b/tests/wpt/tests/css/CSS2/margin-padding-clear/margin-auto-on-block-box-ref.html
new file mode 100644
index 00000000000..efc41cff10c
--- /dev/null
+++ b/tests/wpt/tests/css/CSS2/margin-padding-clear/margin-auto-on-block-box-ref.html
@@ -0,0 +1,54 @@
+
+CSS Reftest Reference
+
+ XXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+
\ No newline at end of file
diff --git a/tests/wpt/tests/css/CSS2/margin-padding-clear/margin-auto-on-block-box.html b/tests/wpt/tests/css/CSS2/margin-padding-clear/margin-auto-on-block-box.html
new file mode 100644
index 00000000000..1afc98a9132
--- /dev/null
+++ b/tests/wpt/tests/css/CSS2/margin-padding-clear/margin-auto-on-block-box.html
@@ -0,0 +1,78 @@
+
+CSS Test: Resolution of margin-left or margin-right set to auto on a non-replaced block box
+
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