feat(compiler): remove whitespace from static class attributes

packages/compiler-core/__tests__/parse.spec.ts

@@ -1015,6 +1015,71 @@ describe('compiler: parse', () => {
       })
     })
 
+    // https://github.com/vuejs/vue-next/issues/4251
+    test('class attribute should ignore whitespace when parsed', () => {
+      const ast = baseParse('<div class=" \n\t c \t\n "></div>')
+      const element = ast.children[0] as ElementNode
+
+      expect(element).toStrictEqual({
+        children: [],
+        codegenNode: undefined,
+        isSelfClosing: false,
+        loc: {
+          end: {
+            column: 10,
+            line: 3,
+            offset: 29
+          },
+          source: '<div class=" \n\t c \t\n "></div>',
+          start: {
+            column: 1,
+            line: 1,
+            offset: 0
+          }
+        },
+        ns: 0,
+        props: [
+          {
+            loc: {
+              end: {
+                column: 3,
+                line: 3,
+                offset: 22
+              },
+              source: 'class=" \n\t c \t\n "',
+              start: {
+                column: 6,
+                line: 1,
+                offset: 5
+              }
+            },
+            name: 'class',
+            type: 6,
+            value: {
+              content: 'c',
+              loc: {
+                end: {
+                  column: 3,
+                  line: 3,
+                  offset: 22
+                },
+                source: '" \n\t c \t\n "',
+                start: {
+                  column: 12,
+                  line: 1,
+                  offset: 11
+                }
+              },
+              type: 2
+            }
+          }
+        ],
+        tag: 'div',
+        tagType: 0,
+        type: 1
+      })
+    })
+
     test('directive with no value', () => {
       const ast = baseParse('<div v-if/>')
       const directive = (ast.children[0] as ElementNode).props[0]

packages/compiler-core/src/parse.ts

@@ -715,7 +715,16 @@ function parseAttributes(
       emitError(context, ErrorCodes.END_TAG_WITH_ATTRIBUTES)
     }
 
-    const attr = parseAttribute(context, attributeNames)
+    let attr = parseAttribute(context, attributeNames)
+
+    // Trim whitespace between class
+    // https://github.com/vuejs/vue-next/issues/4251
+    if (attr.name === 'class') {
+      attr = attr as AttributeNode
+      if (attr.value?.content)
+        attr.value!.content = attr.value!.content.replace(/\s+/g, ' ').trim()
+    }
+
     if (type === TagType.Start) {
       props.push(attr)
     }

packages/shared/src/normalizeProp.ts

@@ -15,7 +15,10 @@ export function normalizeStyle(
         : (normalizeStyle(item) as NormalizedStyle)
       if (normalized) {
         for (const key in normalized) {
-          res[key] = normalized[key]
+          res[key.replace(/\s+/g, ' ').trim()] =
+            typeof normalized[key] === 'string'
+              ? (normalized[key] as string).replace(/\s+/g, ' ').trim()
+              : normalized[key]
         }
       }
     }
@@ -80,7 +83,7 @@ export function normalizeClass(value: unknown): string {
       }
     }
   }
-  return res.trim()
+  return res.replace(/\s+/g, ' ').trim()
 }
 
 export function normalizeProps(props: Record<string, any> | null) {