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HTML

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About the <head>

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Insecure Resource

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CSS

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CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

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To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

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JavaScript

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JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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HTML 

              
                  <form>
  <div>
    <label for="email">Email</label>
    <input id="email" type="email" required placeholder="example@example.com" />
  </div>
  <div>
    <label for="alpha">Alphanumeric Only</label>
    <input id="alpha" type="text" pattern="[a-zA-Z0-9]+" title="Only alphanumeric characters allowed" required/>
  </div>
    <div>
    <label for="select">Select</label>
    <select required>
      <option value="">Select option</option>
      <option value="1">Option one</option>
    </select>
  </div>
  <div>
    <label for="password">Password</label>
    <input id="password" type="password" required/>
  </div>
  <input type="submit" value="Log In" />
</form>

<p><strong>Goal:</strong></p>
<ol>
  <li>Only validate inputs on submit
  <li>Leverage browser-level feedback
  <li>Allow input styling off of <code>.error</code> class because <code>input:invalid</code> sorta sucks and triggers on page load.
  <li>Optional: Trigger validation on blur.
</ol>
<p><code>// TODO:</code> Mess around with <code>pattern</code> validation and maybe <code>setCustomValidity(message)</code>.</p>
<p>Tested in Chrome, Firefox, Edge, and Mobile Safari</p>
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CSS 

              
                body {
  padding: 2em;
  line-height: 1.5;
}

div {
  margin-bottom: 1em;
}

label {
  margin-bottom: 0.25em;
  display: block;  
}

// Never Style input:invalid, it sucks. It triggers way too early. Uncomment to see.
// input:invalid {
//   border-color: red;
// }

// Applying an error class means we keep browser level feedback, error handling, and focus management.
input.error,
select.error,
textarea.error {
  outline: none;
  border-color: red;
  border-width: 1px;
  border-style: solid;
  
  // Use CSS to visually restore valid fields
  &:valid {
    border-style: auto;
    border-color: inherit;
  }
}
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JS 

              
                // Doing error handling on form submit won't work here because the validation blocks the submit event from firing.

//Get all the inputs...
const inputs = document.querySelectorAll('input, select, textarea');

// Loop through them...
for(let input of inputs) {
  // Just before submit, the invalid event will fire, let's apply our class there.
  input.addEventListener('invalid', (event) => {
    input.classList.add('error');    
  }, false);
  
  // Optional: Check validity onblur
  input.addEventListener('blur', (event) => {
    input.checkValidity();
  })

}
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999px

Console