From 7df5f9a7c27c06a390908d0da5b86ac94b3037c2 Mon Sep 17 00:00:00 2001
From: Marco Roth <marco.roth@intergga.ch>
Date: Sun, 16 Nov 2025 01:50:15 -0800
Subject: [PATCH 1/2] Formatter: Respect Outlook Conditional HTML Comments

---
 .../packages/formatter/src/format-printer.ts  |  8 +++
 .../html/outlook-conditional-comments.test.ts | 51 +++++++++++++++++++
 2 files changed, 59 insertions(+)
 create mode 100644 javascript/packages/formatter/test/html/outlook-conditional-comments.test.ts

diff --git a/javascript/packages/formatter/src/format-printer.ts b/javascript/packages/formatter/src/format-printer.ts
index bf6c0f1cc..aaba6575f 100644
--- a/javascript/packages/formatter/src/format-printer.ts
+++ b/javascript/packages/formatter/src/format-printer.ts
@@ -1179,6 +1179,14 @@ export class FormatPrinter extends Printer {
         }
       }).join("")
 
+      const isConditionalComment = /^\[if\s*[^\]]*\]>/.test(inner.trim())
+
+      if (isConditionalComment) {
+        this.pushWithIndent(open + inner + close)
+
+        return
+      }
+
       const hasNewlines = inner.includes('\n')
 
       if (hasNewlines) {
diff --git a/javascript/packages/formatter/test/html/outlook-conditional-comments.test.ts b/javascript/packages/formatter/test/html/outlook-conditional-comments.test.ts
new file mode 100644
index 000000000..ec2487ced
--- /dev/null
+++ b/javascript/packages/formatter/test/html/outlook-conditional-comments.test.ts
@@ -0,0 +1,51 @@
+import { describe, test, expect, beforeAll } from "vitest"
+import { Herb } from "@herb-tools/node-wasm"
+import { Formatter } from "../../src"
+import dedent from "dedent"
+
+let formatter: Formatter
+
+describe("Outlook conditional comments", () => {
+  beforeAll(async () => {
+    await Herb.load()
+
+    formatter = new Formatter(Herb, {
+      indentWidth: 2,
+      maxLineLength: 80
+    })
+  })
+
+  test("preserves conditional comment syntax on same line - issue #877", () => {
+    const source = dedent`
+      <!--[if mso]>
+      <center>
+      <![endif]-->
+    `
+    const result = formatter.format(source)
+    expect(result).toEqual(source)
+  })
+
+  test("handles conditional comment with content", () => {
+    const source = dedent`
+      <!--[if mso]>
+      <table width="100%" cellpadding="0" cellspacing="0">
+        <tr>
+          <td>Content</td>
+        </tr>
+      </table>
+      <![endif]-->
+    `
+    const result = formatter.format(source)
+    expect(result).toEqual(source)
+  })
+
+  test("handles negated conditional comment", () => {
+    const source = dedent`
+      <!--[if !mso]><!-->
+      <div>Not Outlook</div>
+      <!--<![endif]-->
+    `
+    const result = formatter.format(source)
+    expect(result).toEqual(source)
+  })
+})

From 6d671525cbd4a9aa3955f7624f925a8320847706 Mon Sep 17 00:00:00 2001
From: Marco Roth <marco.roth@intergga.ch>
Date: Sun, 16 Nov 2025 02:03:07 -0800
Subject: [PATCH 2/2] Simpler check and extra ERB test

---
 .../packages/formatter/src/format-printer.ts       |  4 ++--
 .../test/html/outlook-conditional-comments.test.ts | 14 ++++++++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/javascript/packages/formatter/src/format-printer.ts b/javascript/packages/formatter/src/format-printer.ts
index aaba6575f..20f8f4678 100644
--- a/javascript/packages/formatter/src/format-printer.ts
+++ b/javascript/packages/formatter/src/format-printer.ts
@@ -1179,9 +1179,9 @@ export class FormatPrinter extends Printer {
         }
       }).join("")
 
-      const isConditionalComment = /^\[if\s*[^\]]*\]>/.test(inner.trim())
+      const trimmedInner = inner.trim()
 
-      if (isConditionalComment) {
+      if (trimmedInner.startsWith('[if ') && trimmedInner.endsWith('<![endif]')) {
         this.pushWithIndent(open + inner + close)
 
         return
diff --git a/javascript/packages/formatter/test/html/outlook-conditional-comments.test.ts b/javascript/packages/formatter/test/html/outlook-conditional-comments.test.ts
index ec2487ced..08b83a257 100644
--- a/javascript/packages/formatter/test/html/outlook-conditional-comments.test.ts
+++ b/javascript/packages/formatter/test/html/outlook-conditional-comments.test.ts
@@ -48,4 +48,18 @@ describe("Outlook conditional comments", () => {
     const result = formatter.format(source)
     expect(result).toEqual(source)
   })
+
+  test("handles conditional comment with ERB interpolation", () => {
+    const source = dedent`
+      <!--[if mso]>
+      <table width="<%= width %>" cellpadding="0" cellspacing="0">
+        <tr>
+          <td><%= content %></td>
+        </tr>
+      </table>
+      <![endif]-->
+    `
+    const result = formatter.format(source)
+    expect(result).toEqual(source)
+  })
 })