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CSS

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HTML 

              
                <div>
    Taille actuelle de la fenêtre : 
  <span id="window_size"></span> px
</div>
<br />
<div id="dynamic" class="resize fresize">
</div>

<pre>
*****************
*** PRÉAMBULE ***
*****************

- les variables représentant la taille (résolution et éléments) sont sans unités 
(pixels) car les calculs dans clamp() (et calc()) sont soumis à certaines règles :

- On ne peut diviser que par un nombre (et non une unité).
- Un côté d'une multiplication doit être un nombre (et non une unité).
- On peut facilement affecter une unité à un nombre en le multipliant par 1 pixel.

*********************
*** MISE EN PLACE ***
********************* 

La tranche de résolution sur laquelle on travaille est placée au niveau du root 
(plus haut niveau du DOM).
La formule est mise en place dans une classe (.resize) pour éviter une redondance 
dans le code CSS. Une 2nde classe (.fresize) est créé spécifiquement pour la police 
car on ne peut pas à la fois cibler l'élément et la police au sein d'un sélecteur CSS.

Concernant l'élement ciblé, on va utiliser la propriété aspect-ratio pour préserver 
le ratio et donc cibler uniquement la largeur de l'élément.
Les variables --min, --max, --fmin et fmax sont définies dans le scope de l'élément.
Elles sont donc réutilisables dans n'importe quel autre élément sans conflit.

******************
*** LA FORMULE ***
******************

(Taille minimale de l'élément) + {(ratio d'étirement) * (étirement maximum de l'élément)}

Taille minimale de l'élément : 100px dans l'exemple
Étirement maximum de l'élément : 100 px (taille maximum - taille minimum)
Ratio d'étirement : (étirement de la fenêtre depuis 900px) / (étirement pamamétré soit 300px (1200px - 900px))
Par exemple, si on a une fenêtre de 1100px, on a étiré de 200px, soit 2/3 
de l'étirement de fenêtre paramétré. On étire donc notre élémment de 2/3 de 100px, soit 67px.

L'utilisation de clamp(valeur minimale, formule, valeur maximale) permet de contenir les 
valeurs de formules hors normes (valeurs négatives ou taille de fenêtre supérieure aux paramètres).

</pre>
!

CSS 

              
                :root {
  --reso_min: 900;
  --reso_max: 1200;
}
.resize {
  --resize: clamp(var(--min) * 1px,
  (var(--min) * 1px) + 
  (((100vw - (var(--reso_min) * 1px)) / (var(--reso_max) - var(--reso_min))) * (var(--max) - var(--min))), var(--max) * 1px)
}
.fresize {
  --fresize: clamp(var(--fmin) * 1px,
  (var(--fmin) * 1px) + 
  (((100vw - (var(--reso_min) * 1px)) / (var(--reso_max) - var(--reso_min))) * (var(--fmax) - var(--fmin))), var(--fmax) * 1px)
}

#dynamic {
  display: flex;
  align-items: center; justify-content: center;
  font-weight: 700;
   background: green;
  
  aspect-ratio: 1 / 1;
  --min: 100;
  --max: 200;
  width: var(--resize);
  --fmin: 10;
  --fmax: 20;
  font-size: var(--fresize);
}
!

JS 

              
                const window_size = document.getElementById('window_size');
const dynamic = document.getElementById('dynamic');

window_size.textContent = window.innerWidth;
dynamic.textContent = dynamic.clientWidth + ' px';

window.addEventListener('resize', function() {
  window_size.textContent = window.innerWidth;
  dynamic.textContent = dynamic.clientWidth + ' px';
});
!
999px

Console