项目的CSS帮助。页面需要有一个"form"元素，当窗口宽于600px时，该元素为600px;当屏幕宽度小于600px时，该元素为屏幕宽度。
当屏幕超过600px时似乎可以工作，但当屏幕收缩到600px以下时不会停留在屏幕的宽度。

.card {
  height: 335px;
  width: 344px;
  border: lightgrey;
  border-style: solid;
  margin-left: auto;
  margin-right: auto;
  font-family: Arial, Helvetica, sans-serif;
  transition: box-shadow;
}

.card:hover {
  box-shadow: 0 2px 4px 0 rgba(0, 0, 0, 0.3);
}

.card-body {
  padding: 16px;
  font-size: 11px;
  color: #232f34;
}

.card_image {
  width: 100%;
  height: 194px;
}

.person {
  height: 50px;
  border-radius: 40px;
  float: left;
  margin: 0 1rem;
}

.title {
  display: inline;
  color: #000;
  font-size: 22px;
  padding: 16px;
}

.secondary {
  color: #232f34;
  display: inline;
  padding: 16px;
}
.myform {
  max-width: 600px;
}

@media screen {
  nav {
    position: static;
    height: auto;
  }
}
@media screen and (min-width: 600px) {
  form {
    display: grid;
    grid-template-columns: 600px;
  }
}

<form method="post" action="/pets">
  <label for="name">Name</label>
  <input name="pet_name" id="name" type="text" />
  <br>
  <label for="type">Type</label>
  <select name="pet_type" id="type">
    <option value="Cat">Cat</option>
    <option value="Dog">Dog</option>
    <option value="Hamster">Hamster</option>
    <option value="Other">Other</option>
    <option value="Zebra">Zebra</option>
  </select>
  <br>

  <label for="bio">Biography</label>
  <textarea name="pet_bio" id="bio"></textarea>
  <br>

  <label for="owner-email">Owner's Email</label>
  <input name="pet_owner_email" id="owner-email" type="text" />
  <br>

  <button class="new-pet-submit-button" type="submit">
    Create new pet
  </button>

  <button class="Reset" type="reset">Reset</button>

  <form class='myform'>
    My form!
  </form>

</form>
<br>
<br>
<div class="card">
  <img src="images/desert.jpg" id="desert" class="card_image">
  <img src="images/person-avatar.jpg" class="person">
  <h1 class="title">Title goes here</h1>
  <h3 class="secondary">Secondary text</h3>
  <p class="card-body"> Greyhound divisively hello coldly wonderfully marginally far upon excluding.</p>
</div>