We added a AccessibilityNodeInfo.setHintText() method on Android O (https://developer.android.com/reference/android/view/accessibility/AccessibilityNodeInfo.html#setHintText(java.lang.CharSequence)), which should be filled with a TextView's hint, which in turn is defined (on View.java) as: "Sets the text to be displayed when the text of the TextView is empty, from a resource."

So, mapping that concept to Web, it would make sense to fill this field with the value of the "placeholder" tag, when present. For example, the Twitter login page (https://twitter.com/login?lang=en) defines:
 
<input
      type="text"
      class="text-input email-input js-signin-email"
      name="session[username_or_email]"
      autocomplete="username"
      placeholder="Phone, email, or username"
    />

<input type="password" class="text-input" name="session[password]" placeholder="Password" autocomplete="current-password">

Right now, Chrome is mapping placeholder tag to the AccessibilityNodeInfo.setText(), which affects Android Autofill on P: we're adding a compatibility mode that converts the AccessibilityNodeInfo into a ViewNode, but we sanitize the node's text property before sending it to the autofill service (A.K.A Password Manager), as it could contain PII.

(this is the same as b/77336622)