Canvas要素をFile System Access APIを使いPNGで保存する。
directoryPickerを使えば、pngの連番が30fpsとかで保存可能。

📝FilePickerを使って、１ファイルごと保存する


var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.moveTo(0, 0);
ctx.lineTo(400, 300);
ctx.stroke();

ctx.fillStyle = "#f00";
ctx.beginPath();
ctx.arc(150, 150, 150, 0, 2 * Math.PI);
ctx.fill();


///////////////////////////////////////////////
//https://stackoverflow.com/questions/72615770/store-canvas-screenshot-to-local-filesystem-with-filesystem-api


c.onclick = ()=>{
  console.log("abc")
   c.toBlob(async function(result) {
        console.log(result);
        const options = {
        types: [
            {
            description: 'Images',
            accept: {
                'image/png': ['.png'],
            },
            },
        ],
        suggestedName: 'Screenshot.png',
        };
        imgFileHandle = await window.showSaveFilePicker(options);
        console.log("Save File chosen");
        const writable = await imgFileHandle.createWritable();
        await writable.write(result);
        await writable.close();          
     
   });
}

📂DirectoryPickerを使って、複数ファイル一気に保存する


var c = document.getElementById("myCanvas");
var btn = document.getElementById("btn");
var ctx = c.getContext("2d");


ctx.moveTo(0, 0);
ctx.lineTo(400, 300);
ctx.stroke();

ctx.fillStyle = "#f00";
ctx.beginPath();
ctx.arc(150, 150, 150, 0, 2 * Math.PI);
ctx.fill();


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


let dhandle;
let idx = 0;

btn.onclick = async ()=>{
  console.log("ファイル名インクリメントして保存")
  if(dhandle==null){
   dhandle = await window.showDirectoryPicker()
   dhandle.requestPermission({ writable: true })    
  }
   
  await c.toBlob(async function(result) {
        //console.log(result);
        let fhandle = await dhandle.getFileHandle('filename'+idx+'.png', { create: true });
        idx++;
        const writable = await fhandle.createWritable();
        await writable.write(result);
        await writable.close();          
     
   });
}