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HTML 

              
                <div class="demo-container">
  <p>Click on SIGN UP button to trigger animation</p>
  <div id="demo"></div>
  <a href="https://www.front-kek.com/demos/form-switch" target="_blank">Tutorial with up-to-date code</a>
</div>
!

CSS 

              
                // global rules
* {
  box-sizing: border-box;
  position: relative;
  padding: 0;
  margin: 0;
}

html,
body {
  max-width: 100vw;
  overflow-x: hidden;
}

body {
  min-height: 100vh;
  background: #fff;
  color: #333;
  line-height: 1.5;
  font-family: 'Roboto', -apple-system, BlinkMacSystemFont, Helvetica, Arial, sans-serif;
}

input, button {
  border: none;
  outline: none;
  background: none;
  font-family: inherit;
}

.demo-container {
  display: flex;
  justify-content: center;
  align-items: center;
  flex-direction: column;
  gap: 20px;
  min-height: 100vh;
  padding: 20px;
  background: #ededed;
  border: 1px solid #ccc;
}

.animated-border {
  // allows you to override it with higher level variable
  --bw: var(--border-width, 3px);

  z-index: 2;
  position: absolute;
  inset: 0;
  clip-path: polygon(
    0 0, 100% 0, 100% 100%, 0 100%, 0 0, // first lap
    var(--bw) var(--bw), // init position for second lap
    var(--bw) calc(100% - var(--bw)), // go down
    calc(100% - var(--bw)) calc(100% - var(--bw)), // to the right
    calc(100% - var(--bw)) var(--bw), // up
    var(--bw) var(--bw) // finish lap
  );

  &:before {
    content: '';
    position: absolute;
    left: 50%;
    top: 50%;
    width: 150%;
    padding-bottom: 150%;
    transform: translate(-50%, -50%);
    background: conic-gradient(from 270deg, #ff4800 10%, #dfd902 35%, #20dc68, #0092f4, #da54d8 72% 75%, #ff4800 95%);
    animation: rotateBtnBg 2s linear infinite;
  }
}

// DEMO STYLES
// if reading scss styles with &__ is too problematic, you can always click on the arrow in top right corner and select "View complited CSS" to see final classes and styles

.local-container { // not critical for the demo, just setting boundries for it
  width: 800px;
  max-width: 100%;
}

.demo {
  $demoRef: &; // saving reference in a variable is a good way to avoid issues if you'll decide to rename the main class after and forget to rename it in all places inside
  --switcher-width: 260px;
  --arrow-offset: 30px;
  --anim-time: 1.2s;
  --transition-transform: transform var(--anim-time) ease-in-out;
  --transition-opacity: opacity 0s calc(var(--anim-time) / 2);

  --btn-height: 36px;

  height: 550px; // can be fluid also
  filter: drop-shadow(0 0 10px rgba(0,0,0,0.3)); // it's basically a box-shadow, but can be applied to complex shapes like clip-path, unlike regular box-shadow

  button {
    display: block;
    margin: 0 auto;
    height: var(--btn-height);
    color: #fff;
    font-size: 15px;
    cursor: pointer;
  }

  @mixin switched { // I'm using mixins to apply rules when state class is active, used via "@include switched"
    #{$demoRef}.s--switched & {
      @content;
    }
  }

  &__inner { // inner container is required to make drop-shadow work with clip-path (of nested element), as per https://css-tricks.com/using-box-shadows-and-clip-path-together/ 
    --demoX1: 0;
    --demoX2: calc(100% - var(--arrow-offset));
    --demoX3: 100%;
    --demoX4: calc(100% - var(--arrow-offset));
    --demoX5: 0;
    --demoX6: 0;

    overflow: hidden;
    height: 100%;
    padding-right: var(--switcher-width);
    background: #fff;
    transition: clip-path var(--anim-time) ease-in-out;
    will-change: clip-path;
    // clip the main container to match the arrow shape, otherwise there will be white corners
    clip-path: polygon(var(--demoX1) 0, var(--demoX2) 0, var(--demoX3) 50%, var(--demoX4) 100%, var(--demoX5) 100%, var(--demoX6) 50%);

    @include switched {
      --demoX1: var(--arrow-offset);
      --demoX2: 100%;
      --demoX3: 100%;
      --demoX4: 100%;
      --demoX5: var(--arrow-offset);
      --demoX6: 0;
    }
  }

  &__forms { // this container nests both of our forms and moves during animation from side to side
    height: 100%;
    transition: var(--transition-transform);
    will-change: transform;

    @include switched {
      transform: translateX(var(--switcher-width));
    }
  }

  &__form { // specific form container that swaps opacity and pointer-events during animation for active/inactive forms
    position: absolute;
    inset: 0;
    transition: var(--transition-opacity);

    &:first-child {
      @include switched {
        opacity: 0;
        pointer-events: none;
      }
    }

    &:last-child {
      opacity: 0;
      pointer-events: none;

      @include switched {
        opacity: 1;
        pointer-events: auto;
      }
    }

    &-content { // nested wrapper for form content that forces it to be centered with width of the switcher
      width: var(--switcher-width);
      margin: 0 auto;
    }
  }

  &__switcher {
    // switcher is essentially a top-level overlay, that gets cropped via clip-path to make it look like an arrow with some content inside
    --x1: calc(100% - var(--switcher-width));
    --x2: calc(100% - var(--arrow-offset));
    --x3: 100%;
    --x4: calc(100% - var(--arrow-offset));
    --x5: calc(100% - var(--switcher-width));
    --x6: calc(100% - var(--switcher-width) + var(--arrow-offset));

    z-index: 2;
    overflow: hidden;
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    background-image: url('https://s3-us-west-2.amazonaws.com/s.cdpn.io/142996/sections-3.jpg');
    background-size: cover;
    background-position: center center;
    clip-path: polygon(var(--x1) 0, var(--x2) 0, var(--x3) 50%, var(--x4) 100%, var(--x5) 100%, var(--x6) 50%);
    transition: clip-path var(--anim-time) ease-in-out;
    will-change: clip-path;

    @include switched {
      --x1: var(--arrow-offset);
      --x2: var(--switcher-width);
      --x3: calc(var(--switcher-width) - var(--arrow-offset));
      --x4: var(--switcher-width);
      --x5: var(--arrow-offset);
      --x6: 0;
    }

    &:before { // overlay to make image background darker, which improves readability of the text
      content: '';
      position: absolute;
      left: 0;
      top: 0;
      width: 100%;
      height: 100%;
      background: rgba(0,0,0,0.6);
    }

    &-inner {
      // I'm using this sub-container with full-width to allow animating switcher content position with transforms, instead of left/right, because of performance difference
      // but transforms got severe limitation - relative % values are tied to element's own width/height, not parent's, so that's why we need this sub-container
      // thanks to calc, we can shift it all the way to the left minus switcher width, which will end up an equivalent of animating from left: calc(100% - var(--switcher-width)) to left: 0
      height: 100%;
      transition: var(--transition-transform);
      will-change: transform;

      @include switched {
        transform: translateX(calc((100% - var(--switcher-width)) * -1));
      }
    }

    &-content {
      // our content is always positioned in the same place on right, we are just moving its parent instead, as explained above
      overflow: hidden;
      position: absolute;
      right: 0;
      top: 0;
      display: flex;
      flex-direction: column;
      justify-content: center;
      column-gap: 20px;
      width: var(--switcher-width);
      height: 100%;
    }

    &-text {
      display: flex;
      flex-wrap: nowrap;
      height: 140px;
      color: #fff;
      transition: var(--transition-transform);
      will-change: transform;

      @include switched {
        transform: translateX(-100%);
      }

      > div {
        width: 100%;
        flex-shrink: 0;
        text-align: center;

        // shift text to better match shape of the arrow
        &:first-child {
          padding-left: calc(var(--arrow-offset) + 10px);
          padding-right: calc(var(--arrow-offset) - 10px);
        }

        &:last-child {
          padding-left: calc(var(--arrow-offset) - 10px);
          padding-right: calc(var(--arrow-offset) + 10px);
        }

        h3 {
          margin-bottom: 20px;
        }

        p {
          font-size: 14px;
        }
      }
    }

    &-btn {
      // I will be covering button with animated border in a separate tutorial, stay tuned :)
      --btn-width: 100px;
      --border-width: 3px;

      overflow: hidden;
      width: var(--btn-width);

      @keyframes rotateBtnBg {
        to {
          transform: translate(-50%, -50%) rotate(360deg);
        }
      }

      &-inner {
        z-index: 1;
        position: absolute;
        inset: 0;
        font-weight: 500;

        span {
          display: block;
          height: 100%;
          line-height: var(--btn-height);
          text-align: center;
          text-transform: uppercase;
          transition: var(--transition-transform);
          will-change: transform;

          @include switched {
            transform: translateY(-100%);
          }
        }
      }
    }
  }
}

.form {
  display: flex;
  flex-direction: column;
  align-items: center;
  width: 100%;
  height: 100%;
  padding: 50px 0;
  row-gap: 20px;
  text-align: center;
  transition: transform var(--anim-time, 1.2s);
  will-change: transform;

  &__heading {
    font-size: 20px;
    font-weight: bold;
  }

  &__field {
    width: 100%;

    &-label {
      font-size: 12px;
      color: #cfcfcf;
      text-transform: uppercase;
    }

    &-input {
      display: block;
      width: 100%;
      max-width: 100%;
      margin-top: 5px;
      padding-bottom: 5px;
      font-size: 16px;
      border-bottom: 1px solid rgba(0,0,0,0.4);
      text-align: center;
    }
  }

  &__submit {
    width: 100%;
    background: #d4af7a;
  }
}
!

JS 

              
                import React, { useState } from 'https://esm.sh/react@18.2.0'
import cn from "https://cdn.skypack.dev/classnames@2.3.2";
import ReactDOM from 'https://esm.sh/react-dom@18.2.0'

function Demo() {
  const [switched, setSwitched] = useState(false);
  return (
    <div className="local-container">
      <div className={cn('demo', { 's--switched': switched })}>
        <div className="demo__inner">
          <div className="demo__forms">
            <div className="demo__form">
              <div className="demo__form-content">
                <FakeForm
                  heading="Welcome back"
                  fields={['email', 'password']}
                  submitLabel="Sign in"
                />
              </div>
            </div>
            <div className="demo__form">
              <div className="demo__form-content">
                <FakeForm
                  heading="Time to feel like home"
                  fields={['name', 'email', 'password']}
                  submitLabel="Sign up"
                />
              </div>
            </div>
          </div>
          <div className="demo__switcher">
            <div className="demo__switcher-inner">
              <div className="demo__switcher-content">
                <div className="demo__switcher-text">
                  <div>
                    <h3>New here?</h3>
                    <p>
                      Sign up and discover great amount of new opportunities!
                    </p>
                  </div>
                  <div>
                    <h3>One of us?</h3>
                    <p>
                      If you already has an account, just sign in. We&apos;ve
                      missed you!
                    </p>
                  </div>
                </div>
                <button
                  className="demo__switcher-btn"
                  onClick={() => setSwitched(!switched)}
                >
                  <span className="animated-border" />
                  <span className="demo__switcher-btn-inner">
                    <span>Sign Up</span>
                    <span>Sign In</span>
                  </span>
                </button>
              </div>
            </div>
          </div>
        </div>
      </div>
    </div>
  );
}

interface FakeFormProps {
  heading: string;
  fields: string[];
  submitLabel: string;
}

function FakeForm({ heading, fields, submitLabel }: FakeFormProps) {
  return (
    <form className="form" onSubmit={(e) => e.preventDefault()}>
      <div className="form__heading">{heading}</div>
      {fields.map((field) => (
        <label className="form__field" key={field}>
          <span className="form__field-label">{field}</span>
          <input className="form__field-input" type={field} />
        </label>
      ))}
      <button type="submit" className="form__submit">
        {submitLabel}
      </button>
    </form>
  );
}

ReactDOM.render(<Demo />, document.querySelector('#demo'));
!
999px

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