Can we set up a preloader with flexdashboard?

Is there any way to add CSS/HTML code to flexdashboard that would be executed just prior to page load, i.e. to set up a preloader?

For instance, if I try any kind of script in rmarkdown's html_output, it's functioning. However, as soon as I switch to flex_dashboard it's not rendering anymore.

Is there any way around this without tinkering with the source code?

Example:

---
output: 
  flexdashboard::flex_dashboard
---
<style>

@import url(https://fonts.googleapis.com/css?family=Open+Sans:300);
body {
  background-color: #f1c40f;
  overflow: hidden;
}

h1 {
  position: absolute;
  font-family: 'Open Sans';
  font-weight: 600;
  font-size: 12px;
  text-transform: uppercase;
  left: 50%;
  top: 58%;
  margin-left: -20px;
}

.body {
  position: absolute;
  top: 50%;
  margin-left: -50px;
  left: 50%;
  animation: speeder .4s linear infinite;
}
.body > span {
  height: 5px;
  width: 35px;
  background: #000;
  position: absolute;
  top: -19px;
  left: 60px;
  border-radius: 2px 10px 1px 0;
}

.base span {
  position: absolute;
  width: 0;
  height: 0;
  border-top: 6px solid transparent;
  border-right: 100px solid #000;
  border-bottom: 6px solid transparent;
}
.base span:before {
  content: "";
  height: 22px;
  width: 22px;
  border-radius: 50%;
  background: #000;
  position: absolute;
  right: -110px;
  top: -16px;
}
.base span:after {
  content: "";
  position: absolute;
  width: 0;
  height: 0;
  border-top: 0 solid transparent;
  border-right: 55px solid #000;
  border-bottom: 16px solid transparent;
  top: -16px;
  right: -98px;
}

.face {
  position: absolute;
  height: 12px;
  width: 20px;
  background: #000;
  border-radius: 20px 20px 0 0;
  transform: rotate(-40deg);
  right: -125px;
  top: -15px;
}
.face:after {
  content: "";
  height: 12px;
  width: 12px;
  background: #000;
  right: 4px;
  top: 7px;
  position: absolute;
  transform: rotate(40deg);
  transform-origin: 50% 50%;
  border-radius: 0 0 0 2px;
}

.body > span > span:nth-child(1),
.body > span > span:nth-child(2),
.body > span > span:nth-child(3),
.body > span > span:nth-child(4) {
  width: 30px;
  height: 1px;
  background: #000;
  position: absolute;
  animation: fazer1 .2s linear infinite;
}

.body > span > span:nth-child(2) {
  top: 3px;
  animation: fazer2 .4s linear infinite;
}

.body > span > span:nth-child(3) {
  top: 1px;
  animation: fazer3 .4s linear infinite;
  animation-delay: -1s;
}

.body > span > span:nth-child(4) {
  top: 4px;
  animation: fazer4 1s linear infinite;
  animation-delay: -1s;
}

@keyframes fazer1 {
  0% {
    left: 0;
  }
  100% {
    left: -80px;
    opacity: 0;
  }
}
@keyframes fazer2 {
  0% {
    left: 0;
  }
  100% {
    left: -100px;
    opacity: 0;
  }
}
@keyframes fazer3 {
  0% {
    left: 0;
  }
  100% {
    left: -50px;
    opacity: 0;
  }
}
@keyframes fazer4 {
  0% {
    left: 0;
  }
  100% {
    left: -150px;
    opacity: 0;
  }
}
@keyframes speeder {
  0% {
    transform: translate(2px, 1px) rotate(0deg);
  }
  10% {
    transform: translate(-1px, -3px) rotate(-1deg);
  }
  20% {
    transform: translate(-2px, 0px) rotate(1deg);
  }
  30% {
    transform: translate(1px, 2px) rotate(0deg);
  }
  40% {
    transform: translate(1px, -1px) rotate(1deg);
  }
  50% {
    transform: translate(-1px, 3px) rotate(-1deg);
  }
  60% {
    transform: translate(-1px, 1px) rotate(0deg);
  }
  70% {
    transform: translate(3px, 1px) rotate(-1deg);
  }
  80% {
    transform: translate(-2px, -1px) rotate(1deg);
  }
  90% {
    transform: translate(2px, 1px) rotate(0deg);
  }
  100% {
    transform: translate(1px, -2px) rotate(-1deg);
  }
}
.longfazers {
  position: absolute;
  width: 100%;
  height: 100%;
}
.longfazers span {
  position: absolute;
  height: 2px;
  width: 20%;
  background: #000;
}
.longfazers span:nth-child(1) {
  top: 20%;
  animation: lf .6s linear infinite;
  animation-delay: -5s;
}
.longfazers span:nth-child(2) {
  top: 40%;
  animation: lf2 .8s linear infinite;
  animation-delay: -1s;
}
.longfazers span:nth-child(3) {
  top: 60%;
  animation: lf3 .6s linear infinite;
}
.longfazers span:nth-child(4) {
  top: 80%;
  animation: lf4 .5s linear infinite;
  animation-delay: -3s;
}

@keyframes lf {
  0% {
    left: 200%;
  }
  100% {
    left: -200%;
    opacity: 0;
  }
}
@keyframes lf2 {
  0% {
    left: 200%;
  }
  100% {
    left: -200%;
    opacity: 0;
  }
}
@keyframes lf3 {
  0% {
    left: 200%;
  }
  100% {
    left: -100%;
    opacity: 0;
  }
}
@keyframes lf4 {
  0% {
    left: 200%;
  }
  100% {
    left: -100%;
    opacity: 0;
  }
}

</style>

<div class='body'>
  <span>
    <span></span>
    <span></span>
    <span></span>
    <span></span>
  </span>
  <div class='base'>
    <span></span>
    <div class='face'></div>
  </div>
</div>
<div class='longfazers'>
  <span></span>
  <span></span>
  <span></span>
  <span></span>
</div>
<h1>Redirecting</h1>

No repository file, test, or entry point is identified in the issue, so first inspect how flex_dashboard renders embedded HTML and how R Markdown's html_output handles pre-load content. Done means documenting or enabling a supported way to include CSS/HTML or scripts that display a preloader before the dashboard loads, without changing package source.