activate() {
    if (this.instance.dispatch('contentActivate', {
      content: this
    }).defaultPrevented || !this.slide) {
      return;
    }

    if (this.isImageContent() && this.isDecoding && !isSafari()) {
      // add image to slide when it becomes active,
      // even if it's not finished decoding
      this.appendImage();
    } else if (this.isError()) {
      this.load(false, true); // try to reload
    }

    if (this.slide.holderElement) {
      this.slide.holderElement.setAttribute('aria-hidden', 'false');
    }
  }
Activate the slide,
active slide is generally the current one,
meaning the user can see it.