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HTML

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HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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Adding Classes

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About the <head>

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Insecure Resource

The resource you are linking to is using the 'http' protocol, which may not work when the browser is using https.

CSS

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

We offer two popular choices: Autoprefixer (which processes your CSS server-side) and -prefix-free (which applies prefixes via a script, client-side).

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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Packages

Add Packages

Search for and use JavaScript packages from npm here. By selecting a package, an import statement will be added to the top of the JavaScript editor for this package.

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Using packages here is powered by esm.sh, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ESM usage.

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HTML 

              
                <input id="start-date" type="date"/>
<div id="debug-log"></div>
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CSS
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JS 

              
                // 'blur' for test...

function logToPage(message) {
  const logContainer = document.getElementById('debug-log')
  if (logContainer) {
    const logEntry = document.createElement('div')
    logEntry.textContent = message
    logContainer.appendChild(logEntry) // Ajoute le message au conteneur
  }
}

document.addEventListener('DOMContentLoaded', () => {
  const startDateInput = document.getElementById('start-date')
  
  startDateInput.addEventListener('blur', function () {
    if (!this.value) {
      return
    }

    const selectedDate = new Date(this.value)

    if (isNaN(selectedDate.getTime())) {
      alert(`Date invalide. Veuillez réessayer.`)
      this.value = ''
      return
    }

    const day = selectedDate.getDay() // .getUTCDay()

    // Vérifiez si le jour sélectionné est un lundi
    if (day !== 1) {
      alert(`Veuillez sélectionner un lundi.`)
      this.value = ''
    } else {
      localStorage.setItem('startDate', this.value)
    }
  })
})

////////////////////////////////////////////////////////////
/*
document.addEventListener('DOMContentLoaded', () => {
  const startDateInput = document.getElementById('start-date')

  // Gestionnaire d'événement pour la date de début
  startDateInput.addEventListener('input', function () {
    if (!this.value) {
      return;
    }
    
    const selectedDate = new Date(this.value)
    
    if (isNaN(selectedDate.getTime())) {
      alert(`Date invalide. Veuillez réessayer.`)
      this.value = ''
      return
    }
    
    const day = selectedDate.getUTCDay()

    // Vérifiez si le jour sélectionné est un lundi (1 dans getDay())
    if (day !== 1) {
      alert(`Veuillez sélectionner un lundi.`)
      this.value = ''
    } else {
      localStorage.setItem('startDate', this.value)
    }
  })
})

////////////////////////////////////////////////////////

document.addEventListener('DOMContentLoaded', () => {
  const startDateInput = document.getElementById('start-date')

  // Gestionnaire d'événement pour la date de début
  startDateInput.addEventListener('input', function () {
    if (!this.value) {
      return;
    }
    
    const selectedDate = new Date(this.value)
    
    if (isNaN(selectedDate.getTime())) {
      alert(`Date invalide. Veuillez réessayer.`)
      this.value = ''
      return
    }
    
    const day = selectedDate.getUTCDay()

    // Vérifiez si le jour sélectionné est un lundi (1 dans getDay())
    if (day !== 1) {
      alert(`Veuillez sélectionner un lundi.`)
      this.value = ''
    } else {
      localStorage.setItem('startDate', this.value)
    }
  })
})

////////////////////////////////////////////////////////////////
document.addEventListener('DOMContentLoaded', () => {
  const startDateInput = document.getElementById('start-date')
  
  function isIphone() { // @bugfix Filtrage des iPhones en raison d'un bug : 0 par défaut, puis accepte la valeur 1 pour le lundi ; pas de solution pour l'instant.
    return /iPhone|iPad|iPod/i.test(navigator.userAgent)
  }

  // Gestionnaire d'événement pour la date de début
  startDateInput.addEventListener('input', function () {
    if (!this.value) {
      return
    }

    if (isIphone()) { // @todo Solution à revoir.
      return
    }

    const selectedDate = new Date(this.value)

    if (isNaN(selectedDate.getTime())) {
      alert(`Date invalide. Veuillez réessayer.`)
      this.value = ''
      return
    }

    const day = selectedDate.getUTCDay()

    // Vérifiez si le jour sélectionné est un lundi
    if (day !== 1) {
      alert(`Veuillez sélectionner un lundi.`)
      this.value = ''
    } else {
      localStorage.setItem('startDate', this.value)
    }
  })
})
*/
!
999px

Console