editor.js/src/styles/block.css

@keyframes fade-in {
    from {
        opacity: 0;
    }

    to {
        opacity: 1;
    }
}

.ce-block {
    animation: fade-in 300ms ease;
    animation-fill-mode: initial;

    &:first-of-type {
        margin-top: 0;
    }

    &--selected &__content {
        background: var(--selectionColor);

        /**
     * Workaround Safari case when user can select inline-fragment with cross-block-selection
     */
        & [contenteditable] {
            -webkit-user-select: none;
            user-select: none;
        }

        img,
        .ce-stub {
            opacity: 0.55;
        }
    }

    &--stretched &__content {
        max-width: none;
    }

    &__content {
        position: relative;
        max-width: var(--content-width);
        margin: 0 auto;
        transition: background-color 150ms ease;
    }

    a {
        cursor: pointer;
        text-decoration: underline;
    }

    b {
        font-weight: bold;
    }

    i {
        font-style: italic;
    }
}