diff --git a/MarkupString.Html/Emitters/LayoutHtml.cs b/MarkupString.Html/Emitters/LayoutHtml.cs
index 4169b22..9c3d5ab 100644
--- a/MarkupString.Html/Emitters/LayoutHtml.cs
+++ b/MarkupString.Html/Emitters/LayoutHtml.cs
@@ -107,11 +107,14 @@ private static void Rule(Rule rule, string kind, BorderStyle? border, HtmlLayout
 
 	private static void Flex(Flex flex, HtmlLayoutWriter html)
 	{
+		var items = flex.Items.IsDefault ? [] : flex.Items;
+		var spacing = flex.Separator is { } separator ? separator.DisplayWidth : Math.Max(0, flex.Gap);
+		var reserved = flex.Vertical ? 0 : spacing * Math.Max(0, items.Length - 1);
 		html.Write("<div class=\"ms-flex");
 		if (flex.Separator is not null) html.Write(" ms-divided");
 		if (flex.Vertical) html.Write(" ms-vertical");
 		html.Write("\" style=\"column-gap:");
-		html.Write(Number(flex.Separator is { } separator ? separator.DisplayWidth : Math.Max(0, flex.Gap)));
+		html.Write(Number(spacing));
 		html.Write("ch");
 		if (flex.Justify != FlexJustify.Start)
 		{
@@ -130,7 +133,7 @@ private static void Flex(Flex flex, HtmlLayoutWriter html)
 		}
 		html.Write("\">");
 
-		foreach (var block in flex.Items.IsDefault ? [] : flex.Items)
+		foreach (var block in items)
 		{
 			// Each item asks for its terminal width and grows to fill its row, so items that do not fit
 			// side by side wrap onto rows of their own. Spare width is left alone only when the layout
@@ -145,6 +148,8 @@ private static void Flex(Flex flex, HtmlLayoutWriter html)
 			html.Write(item.Basis.Kind switch
 			{
 				BlockSizeKind.Cells => Number(item.Basis.Value) + "ch",
+				BlockSizeKind.Percent when reserved > 0 && item.Basis.Value > 0 =>
+					"calc(" + Number(item.Basis.Value) + "% - " + Decimal(reserved * item.Basis.Value / 100.0) + "ch)",
 				BlockSizeKind.Percent => Number(item.Basis.Value) + "%",
 				_ => item.Min > 1 ? Number(item.Min) + "ch" : "0",
 			});
diff --git a/MarkupString.Html/LayoutCss.cs b/MarkupString.Html/LayoutCss.cs
index 2354df8..f983a2d 100644
--- a/MarkupString.Html/LayoutCss.cs
+++ b/MarkupString.Html/LayoutCss.cs
@@ -38,7 +38,7 @@ public static class LayoutCss
 		".ms-border-none.ms-rule::before, .ms-border-none.ms-rule::after { border-top: none; }\n" +
 		".ms-flex { display: flex; flex-wrap: wrap; }\n" +
 		".ms-flex.ms-vertical { flex-direction: column; }\n" +
-		".ms-item { min-width: 0; }\n" +
+		".ms-item { min-width: 0; box-sizing: border-box; }\n" +
 		".ms-flex.ms-divided > .ms-item + .ms-item { border-left: 1px solid var(--ms-separator, var(--ms-separator-default, currentColor)); padding-left: 1ch; }\n" +
 		".ms-figure { display: flow-root; }\n" +
 		".ms-figure-image { max-width: 100%; height: auto; }\n" +
diff --git a/MarkupString.Tests/Layout/BlockLayoutTests.cs b/MarkupString.Tests/Layout/BlockLayoutTests.cs
index 4f2087d..1f6483a 100644
--- a/MarkupString.Tests/Layout/BlockLayoutTests.cs
+++ b/MarkupString.Tests/Layout/BlockLayoutTests.cs
@@ -238,6 +238,22 @@ public async Task Flex_AutoItemsShareTheRest()
 		await Assert.That(BlockLayout.Lines(flex, 20)[0].ToPlainText()).IsEqualTo("a    b     c        ");
 	}
 
+	[Test]
+	public async Task Flex_PercentageItemsLeaveRoomForTheirSeparatorInHtml()
+	{
+		var flex = new Flex(
+		[
+			T("left").Sized(BlockSize.Percent(50)),
+			T("right").Sized(BlockSize.Percent(50)),
+		])
+		{ Separator = P(" │ ") };
+
+		var html = BlockLayout.Build(flex, 40).Render(MarkupFormat.Html, Registry);
+
+		await Assert.That(html).Contains("flex:1 1 calc(50% - 1.5ch)");
+		await Assert.That(LayoutCss.Fixed).Contains(".ms-item { min-width: 0; box-sizing: border-box; }");
+	}
+
 	[Test]
 	public async Task Figure_TextFlowsRoundTheArt()
 	{