HTML
<div class="container">
<form>
<ul class="flex-outer">
<li>
<label for="first-name">First Name</label>
<input type="text" id="first-name" placeholder="Enter your first name here">
</li>
<li>
<label for="last-name">Last Name</label>
<input type="text" id="last-name" placeholder="Enter your last name here">
</li>
<li>
<label for="email">Email</label>
<input type="email" id="email" placeholder="Enter your email here">
</li>
<li>
<label for="phone">Phone</label>
<input type="tel" id="phone" placeholder="Enter your phone here">
</li>
<li>
<label for="message">Message</label>
<textarea rows="6" id="message" placeholder="Enter your message here"></textarea>
</li>
<li>
<p>Age</p>
<ul class="flex-inner">
<li>
<input type="checkbox" id="twenty-to-twentynine">
<label for="twenty-to-twentynine">20-29</label>
</li>
<li>
<input type="checkbox" id="thirty-to-thirtynine">
<label for="thirty-to-thirtynine">30-39</label>
</li>
<li>
<input type="checkbox" id="fourty-to-fourtynine">
<label for="fourty-to-fourtynine">40-49</label>
</li>
<li>
<input type="checkbox" id="fifty-to-fiftynine">
<label for="fifty-to-fiftynine">50-59</label>
</li>
<li>
<input type="checkbox" id="sixty-to-sixtynine">
<label for="sixty-to-sixtynine">60-69</label>
</li>
<li>
<input type="checkbox" id="other">
<label for="other">Other</label>
</li>
</ul>
</li>
<li>
<button type="submit">Submit</button>
</li>
</ul>
</form>
</div>
CSS
body {
font: normal 18px/1.5 "Fira Sans", "Helvetica Neue", sans-serif;
background: #70C1B3;
color: #fff;
padding: 50px 0;
}
.container {
width: 80%;
max-width: 1200px;
margin: 0 auto;
}
.container * {
box-sizing: border-box;
}
.flex-outer,
.flex-inner {
list-style-type: none;
padding: 0;
}
.flex-outer {
max-width: 800px;
margin: 0 auto;
}
.flex-outer li,
.flex-inner {
display: flex;
flex-wrap: wrap;
align-items: center;
}
.flex-inner {
padding: 0 8px;
justify-content: space-between;
}
.flex-outer > li:not(:last-child) {
margin-bottom: 20px;
}
.flex-outer li label,
.flex-outer li p {
padding: 8px;
font-weight: 300;
letter-spacing: .09em;
text-transform: uppercase;
}
.flex-outer > li > label,
.flex-outer li p {
flex: 1 0 120px;
max-width: 220px;
}
.flex-outer > li > label + *,
.flex-inner {
flex: 1 0 220px;
}
.flex-outer li p {
margin: 0;
}
.flex-outer li input:not([type='checkbox']),
.flex-outer li textarea {
padding: 15px;
border: none;
}
.flex-outer li button {
margin-left: auto;
padding: 8px 16px;
border: none;
background: #333;
color: #f2f2f2;
text-transform: uppercase;
letter-spacing: .09em;
border-radius: 2px;
}
.flex-inner li {
width: 100px;
}
