<button class="btn--angle"><span class="btn__inner">An angle button</span></button>

<hr>

<button class="btn--angle"><span class="btn__inner">An angle button with a longer name just fails</span></button>

/**
 * Button angle with pseudo-element
 */


.btn--angle {
    --angle-width: 2rem;
    --bg-color: lightseagreen;
    --bg-color-hover: steelblue;
    --duration: 300ms;
    --easing: cubic-bezier(0.165, 0.840, 0.440, 1);
    --padding-x: 1rem;
    --padding-y: 1rem;
    
    background-color: transparent;
    color: black;
    display: inline-block;
    line-height: 1;
    padding-right: var(--angle-width);
    position: relative;
}

.btn--angle::before {
    content: '';
    border: 0 solid transparent;
    border-bottom-width: calc(var(--angle-width) * 0.75);
    border-top-width: calc(var(--angle-width) * 0.75);
    border-left: var(--angle-width) solid var(--bg-color);
    position: absolute;
    right: 0;
    top: 0;
    transition: border-color var(--duration) var(--easing);
}

.btn--angle .btn__inner {
    background-color: var(--bg-color);
    display: block;
    padding: var(--padding-y) var(--padding-x);
    transition: background-color var(--duration) var(--easing);
}

.btn--angle:hover::before {
    border-left-color: var(--bg-color-hover);
    transition: border-color var(--duration) var(--easing);
}

.btn--angle:hover .btn__inner {
    background-color: var(--bg-color-hover);
    transition: background-color var(--duration) var(--easing);
}


/**
 * demo styles
 */

* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

body {
    background-color: ghostwhite;
    font-family: system-ui, -apple-system, sans-serif;
    line-height: 1.5;
    max-width: 250px;
    padding: 1rem;
}

button {
    border: 0;
    cursor: pointer;
    font-family: system-ui, -apple-system, sans-serif;
    font-weight: 600;
    text-align: left;
}

hr {
    border: 0;
    margin: 2rem;
}