diff --git a/e2e/contact.e2e.ts b/e2e/contact.e2e.ts
index 01757f7..7be2e8d 100644
--- a/e2e/contact.e2e.ts
+++ b/e2e/contact.e2e.ts
@@ -18,6 +18,10 @@ test.describe('contact briefing', () => {
     await page.goto(pagePath('contact', 'de'), { waitUntil: 'networkidle' });
     extras.length = 0;
 
+    const emptySubmit = page.locator('.submit');
+    await expect(emptySubmit).toHaveAttribute('aria-disabled', 'true');
+    expect(await emptySubmit.getAttribute('href')).toBeNull();
+
     await page.locator('#contact-name').fill('Ada');
     await page.locator('#contact-email').fill('ada@example.com');
     await page.locator('#contact-projectType').selectOption('software');
diff --git a/src/app/shared/contact-briefing/contact-briefing.html b/src/app/shared/contact-briefing/contact-briefing.html
index a3b962b..6668621 100644
--- a/src/app/shared/contact-briefing/contact-briefing.html
+++ b/src/app/shared/contact-briefing/contact-briefing.html
@@ -18,9 +18,11 @@
             [value]="values()[field.id]"
             [required]="field.required"
             [attr.aria-required]="field.required ? 'true' : null"
-            [attr.aria-describedby]="field.hint ? controlId(field) + '-hint' : null"
+            [attr.aria-invalid]="showInvalid(field) ? 'true' : null"
+            [attr.aria-describedby]="describedBy(field)"
             rows="6"
             (input)="onInput(field.id, $event)"
+            (blur)="onBlur(field.id)"
           ></textarea>
         }
         @case ('select') {
@@ -29,8 +31,10 @@
             [value]="values()[field.id]"
             [required]="field.required"
             [attr.aria-required]="field.required ? 'true' : null"
-            [attr.aria-describedby]="field.hint ? controlId(field) + '-hint' : null"
+            [attr.aria-invalid]="showInvalid(field) ? 'true' : null"
+            [attr.aria-describedby]="describedBy(field)"
             (input)="onInput(field.id, $event)"
+            (blur)="onBlur(field.id)"
           >
             <option value=""></option>
             @for (option of field.options ?? []; track option.value) {
@@ -45,20 +49,34 @@
             [value]="values()[field.id]"
             [required]="field.required"
             [attr.aria-required]="field.required ? 'true' : null"
-            [attr.aria-describedby]="field.hint ? controlId(field) + '-hint' : null"
+            [attr.aria-invalid]="showInvalid(field) ? 'true' : null"
+            [attr.aria-describedby]="describedBy(field)"
             (input)="onInput(field.id, $event)"
+            (blur)="onBlur(field.id)"
           />
         }
       }
     </div>
   }
 
-  <p class="incomplete" aria-live="polite">{{ incompleteMessage() }}</p>
+  <p class="incomplete" [id]="incompleteId" aria-live="polite">{{ incompleteMessage() }}</p>
 
   <p class="note">{{ copy().noBackendNote }}</p>
 
   <p class="cluster actions">
-    <a class="submit" [href]="mailtoHref()">{{ copy().submitLabel }}</a>
+    @if (isComplete()) {
+      <a class="submit" [href]="mailtoHref()">{{ copy().submitLabel }}</a>
+    } @else {
+      <button
+        type="button"
+        class="submit"
+        aria-disabled="true"
+        [attr.aria-describedby]="incompleteId"
+        (click)="onDisabledSubmit($event)"
+      >
+        {{ copy().submitLabel }}
+      </button>
+    }
     <a [href]="'mailto:' + contactEmail">{{ copy().directEmailLabel }}</a>
     @if (showCalendar()) {
       <a [href]="calendarUrl">{{ copy().calendarLabel }}</a>
diff --git a/src/app/shared/contact-briefing/contact-briefing.scss b/src/app/shared/contact-briefing/contact-briefing.scss
index 67790f2..c6a75a7 100644
--- a/src/app/shared/contact-briefing/contact-briefing.scss
+++ b/src/app/shared/contact-briefing/contact-briefing.scss
@@ -31,7 +31,8 @@ textarea {
   font: inherit;
 }
 
-.actions a {
+.actions a,
+button.submit {
   color: var(--color-accent-cool);
   text-decoration: none;
 }
@@ -40,6 +41,21 @@ textarea {
   font-weight: 600;
 }
 
+button.submit {
+  appearance: none;
+  border: 0;
+  padding: 0;
+  background: transparent;
+  font: inherit;
+  font-weight: 600;
+  min-height: 2.75rem;
+}
+
+button.submit[aria-disabled='true'] {
+  color: var(--color-text-muted);
+  cursor: not-allowed;
+}
+
 @media (hover: hover) and (pointer: fine) {
   .actions a:hover {
     text-decoration: underline;
diff --git a/src/app/shared/contact-briefing/contact-briefing.spec.ts b/src/app/shared/contact-briefing/contact-briefing.spec.ts
index a7f1075..3f14fa4 100644
--- a/src/app/shared/contact-briefing/contact-briefing.spec.ts
+++ b/src/app/shared/contact-briefing/contact-briefing.spec.ts
@@ -39,6 +39,12 @@ describe('ContactBriefing', () => {
     expect(root.querySelector('form[action]')).toBeNull();
     expect(root.textContent).toContain(copy.incompleteHint);
     expect(root.querySelector('[aria-live="polite"]')).toBeTruthy();
+    expect(root.querySelector('a.submit')).toBeNull();
+    const disabledSubmit = root.querySelector<HTMLButtonElement>('button.submit');
+    expect(disabledSubmit?.getAttribute('aria-disabled')).toBe('true');
+    expect(disabledSubmit?.getAttribute('aria-describedby')).toBe('contact-incomplete');
+    expect(disabledSubmit?.textContent).toContain(copy.submitLabel);
+    expect(root.querySelector('[aria-invalid="true"]')).toBeNull();
     expect(root.querySelector(`a[href="${SITE_CONFIG.calendarUrl}"]`)).toBeNull();
     expect(root.textContent).not.toContain(copy.calendarLabel);
 
@@ -85,4 +91,33 @@ describe('ContactBriefing', () => {
     expect(fetchSpy).not.toHaveBeenCalled();
     expect(xhrSpy).not.toHaveBeenCalled();
   });
+
+  it('marks required fields invalid only after interaction', async () => {
+    await TestBed.configureTestingModule({
+      imports: [ContactBriefing],
+    }).compileComponents();
+
+    const fixture = TestBed.createComponent(ContactBriefing);
+    const copy = SITE_CONTENT_DATA.de.contact;
+    fixture.componentRef.setInput('copy', copy);
+    await fixture.whenStable();
+
+    const root = fixture.nativeElement as HTMLElement;
+    const name = root.querySelector<HTMLInputElement>('#contact-name');
+    expect(name).toBeTruthy();
+    expect(name?.getAttribute('aria-invalid')).toBeNull();
+
+    name!.dispatchEvent(new Event('blur', { bubbles: true }));
+    fixture.detectChanges();
+
+    expect(name?.getAttribute('aria-invalid')).toBe('true');
+    expect(name?.getAttribute('aria-describedby')?.includes('contact-incomplete')).toBe(true);
+    expect(root.querySelector('#contact-email')?.getAttribute('aria-invalid')).toBeNull();
+
+    const submit = root.querySelector<HTMLButtonElement>('button.submit');
+    expect(submit?.getAttribute('aria-disabled')).toBe('true');
+    submit?.click();
+    fixture.detectChanges();
+    expect(root.querySelector('a.submit')).toBeNull();
+  });
 });
diff --git a/src/app/shared/contact-briefing/contact-briefing.ts b/src/app/shared/contact-briefing/contact-briefing.ts
index dac1b65..3c47087 100644
--- a/src/app/shared/contact-briefing/contact-briefing.ts
+++ b/src/app/shared/contact-briefing/contact-briefing.ts
@@ -25,8 +25,10 @@ export class ContactBriefing {
   readonly copy = input.required<ContactPageCopy>();
 
   protected readonly values = signal<Record<ContactFieldId, string>>({ ...EMPTY_VALUES });
+  protected readonly touched = signal<Partial<Record<ContactFieldId, boolean>>>({});
   protected readonly contactEmail = SITE_CONFIG.contactEmail;
   protected readonly calendarUrl = SITE_CONFIG.calendarUrl;
+  protected readonly incompleteId = 'contact-incomplete';
 
   protected readonly missingLabels = computed(() => {
     const copy = this.copy();
@@ -36,6 +38,8 @@ export class ContactBriefing {
       .map((field) => field.label);
   });
 
+  protected readonly isComplete = computed(() => this.missingLabels().length === 0);
+
   protected readonly incompleteMessage = computed(() => {
     const missing = this.missingLabels();
     if (missing.length === 0) {
@@ -60,12 +64,44 @@ export class ContactBriefing {
   protected onInput(fieldId: ContactFieldId, event: Event): void {
     const target = event.target as HTMLInputElement | HTMLTextAreaElement | HTMLSelectElement;
     this.values.update((current) => ({ ...current, [fieldId]: target.value }));
+    this.markTouched(fieldId);
+  }
+
+  protected onBlur(fieldId: ContactFieldId): void {
+    this.markTouched(fieldId);
+  }
+
+  protected onDisabledSubmit(event: Event): void {
+    event.preventDefault();
   }
 
   protected controlId(field: ContactFieldCopy): string {
     return `contact-${field.id}`;
   }
 
+  protected showInvalid(field: ContactFieldCopy): boolean {
+    return (
+      field.required &&
+      this.touched()[field.id] === true &&
+      this.values()[field.id].trim().length === 0
+    );
+  }
+
+  protected describedBy(field: ContactFieldCopy): string | null {
+    const ids: string[] = [];
+    if (field.hint) {
+      ids.push(`${this.controlId(field)}-hint`);
+    }
+    if (this.showInvalid(field)) {
+      ids.push(this.incompleteId);
+    }
+    return ids.length > 0 ? ids.join(' ') : null;
+  }
+
+  private markTouched(fieldId: ContactFieldId): void {
+    this.touched.update((current) => ({ ...current, [fieldId]: true }));
+  }
+
   private assembleBody(copy: ContactPageCopy, values: Record<ContactFieldId, string>): string {
     const lines = copy.fields.flatMap((field) => {
       const raw = values[field.id];